ETABS. Integrated Building Design Software. Concrete Frame Design Manual. Computers and Structures, Inc. Berkeley, California, USA


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2 ETABS Integrated Building Design Software Concrete Frame Design Manual Computers and Structures, Inc. Berkeley, California, USA Version 8 January 2002
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4 Copyright The computer program ETABS and all associated documentation are proprietary and copyrighted products. Worldwide rights of ownership rest with Computers and Structures, Inc. Unlicensed use of the program or reproduction of the documentation in any form, without prior written authorization from Computers and Structures, Inc., is explicitly prohibited. Further information and copies of this documentation may be obtained from: Computers and Structures, Inc University Avenue Berkeley, California USA Phone: (510) FAX: (510) (for general questions) (for technical support questions) web: Copyright Computers and Structures, Inc., The CSI Logo is a trademark of Computers and Structures, Inc. ETABS is a trademark of Computers and Structures, Inc. Windows is a registered trademark of Microsoft Corporation. Adobe and Acrobat are registered trademarks of Adobe Systems Incorporated
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6 DISCLAIMER CONSIDERABLE TIME, EFFORT AND EXPENSE HAVE GONE INTO THE DEVELOPMENT AND DOCUMENTATION OF ETABS. THE PROGRAM HAS BEEN THOROUGHLY TESTED AND USED. IN USING THE PROGRAM, HOWEVER, THE USER ACCEPTS AND UNDERSTANDS THAT NO WARRANTY IS EXPRESSED OR IMPLIED BY THE DEVELOPERS OR THE DISTRIBUTORS ON THE ACCURACY OR THE RELIABILITY OF THE PROGRAM. THIS PROGRAM IS A VERY PRACTICAL TOOL FOR THE DESIGN/CHECK OF CONCRETE STRUCTURES. HOWEVER, THE USER MUST THOROUGHLY READ THE MANUAL AND CLEARLY RECOGNIZE THE ASPECTS OF CONCRETE DESIGN THAT THE PROGRAM ALGORITHMS DO NOT ADDRESS. THE USER MUST EXPLICITLY UNDERSTAND THE ASSUMPTIONS OF THE PROGRAM AND MUST INDEPENDENTLY VERIFY THE RESULTS.
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8 COMPUTERS AND STRUCTURES, INC., BERKELEY, CALIFORNIA DECEMBER 2001 CONCRETE FRAME DESIGN Contents General Concrete Frame Design Information 1 General Design Information Design Codes 11 Units 11 Overwriting the Frame Design Procedure for a Concrete 11 Frame Design Load Combinations 12 Design of Beams 12 Design of Columns 13 Beam/Column Flexural Capacity Ratios 14 Second Order PDelta Effects 14 Element Unsupported Lengths 16 Analysis Sections and Design Sections Concrete Frame Design Process Concrete Frame Design Procedure Interactive Concrete Frame Design General 31 Concrete Design Information Form Output Data Plotted Directly on the Model Overview 41 Using the Print Design Tables Form 41 Design Input 42 Design Output 42 i
9 Concrete Frame Design Manual Concrete Frame Design Specific to UBC97 5 General and Notation Introduction to the UBC 97 Series of Technical Notes 51 Notation Preferences General 61 Using the Preferences Form 61 Preferences Overwrites General 71 Overwrites 71 Making Changes in the Overwrites Form 73 Resetting Concrete Frame Overwrites to Default 74 Values 8 Design Load Combinations 9 Strength Reduction Factors 10 Column Design Overview 101 Generation of Biaxial Interaction Surfaces 102 Calculate Column Capacity Ratio 105 Determine Factored Moments and Forces 106 Determine Moment Magnification Factors 106 Determine Capacity Ratio 108 Required Reinforcing Area Design Column Shear Reinforcement Determine Required Shear Reinforcement Reference Beam Design Overview 111 Design Beam Flexural Reinforcement 111 Determine Factored Moments 112 Determine Required Flexural Reinforcement 112 ii
10 Contents Design Beam Shear Reinforcement Joint Design Overview 121 Determine the Panel Zone Shear Force 121 Determine the Effective Area of Joint 125 Check Panel Zone Shear Stress 125 Beam/Column Flexural Capacity Ratios Input Data Input data 131 Using the Print Design Tables Form Output Details Using the Print Design Tables Form 143 Concrete Frame Design Specific to ACI General and Notation Introduction to the ACI Series of Technical 151 Notes Notation Preferences General 161 Using the Preferences Form 161 Preferences Overwrites General 171 Overwrites 171 Making Changes in the Overwrites Form 173 Resetting Concrete Frame Overwrites to Default 174 Values 18 Design Load Combinations 19 Strength Reduction Factors iii
11 Concrete Frame Design Manual 20 Column Design Overview 201 Generation of Biaxial Interaction Surfaces 202 Calculate Column Capacity Ratio 205 Determine Factored Moments and Forces 206 Determine Moment Magnification Factors 206 Determine Capacity Ratio 209 Required Reinforcing Area Design Column Shear Reinforcement Determine Section Forces Determine Concrete Shear Capacity Determine Required Shear Reinforcement References Beam Design Overview 211 Design Beam Flexural Reinforcement 211 Determine Factored Moments 212 Determine Required Flexural Reinforcement 212 Design for TBeam 215 Minimum Tensile Reinforcement 218 Special Consideration for Seismic Design 218 Design Beam Shear Reinforcement 219 Determine Shear Force and Moment Determine Concrete Shear Capacity Determine Required Shear Reinforcement Joint Design Overview 221 Determine the Panel Zone Shear Force 221 Determine the Effective Area of Joint 224 Check Panel Zone Shear Stress 224 Beam/Column Flexural Capacity Ratios Input Data Input Data 231 Using the Print Design Tables Form 233 iv
12 Contents 24 Output Details Using the Print Design Tables Form 243 v
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14 COMPUTERS AND STRUCTURES, INC., BERKELEY, CALIFORNIA JANUARY 2002 CONCRETE FRAME DESIGN Technical Note 1 General Design Information This Technical Note presents some basic information and concepts helpful when performing concrete frame design using this program. Design Codes The design code is set using the Options menu > Preferences > Concrete Frame Design command. You can choose to design for any one design code in any one design run. You cannot design some elements for one code and others for a different code in the same design run. You can, however, perform different design runs using different design codes without rerunning the analysis. Units For concrete frame design in this program, any set of consistent units can be used for input. You can change the system of units at any time. Typically, design codes are based on one specific set of units. Overwriting the Frame Design Procedure for a Concrete Frame The two design procedures possible for concrete beam design are: Concrete frame design No design If a line object is assigned a frame section property that has a concrete material property, its default design procedure is Concrete Frame Design. A concrete frame element can be switched between the Concrete Frame Design and the "None" design procedure. Assign a concrete frame element the "None" design procedure if you do not want it designed by the Concrete Frame Design postprocessor. Design Codes Technical Note 11
15 General Design Information Concrete Frame Design Change the default design procedure used for concrete frame elements by selecting the element(s) and clicking Design menu > Overwrite Frame Design Procedure. This change is only successful if the design procedure assigned to an element is valid for that element. For example, if you select a concrete element and attempt to change the design procedure to Steel Frame Design, the program will not allow the change because a concrete element cannot be changed to a steel frame element. Design Load Combinations The program creates a number of default design load combinations for concrete frame design. You can add in your own design load combinations. You can also modify or delete the program default load combinations. An unlimited number of design load combinations can be specified. To define a design load combination, simply specify one or more load cases, each with its own scale factor. For more information see Concrete Frame Design UBC97 Technical Note 8 Design Load Combination and Concrete Frame Design ACI Technical Note 18 Design Load Combination. Design of Beams The program designs all concrete frame elements designated as beam sections in their Frame Section Properties as beams (see Define menu >Frame Sections command and click the Reinforcement button). In the design of concrete beams, in general, the program calculates and reports the required areas of steel for flexure and shear based on the beam moments, shears, load combination factors, and other criteria, which are described in detail in Concrete Frame UBC97 Technical Note Beam Design 11 and Concrete Frame ACI Technical Note 21 Beam Design. The reinforcement requirements are calculated at each output station along the beam span. All the beams are designed for major direction flexure and shear only. Effects resulting from any axial forces, minor direction bending, and torsion that may exist in the beams must be investigated independently by the user. In designing the flexural reinforcement for the major moment at a particular section of a particular beam, the steps involve the determination of the maximum factored moments and the determination of the reinforcing steel. Technical Note 12 Design Load Combinations
16 Concrete Frame Design General Design Information The beam section is designed for the maximum positive and maximum negative factored moment envelopes obtained from all of the load combinations. Negative beam moments produce top steel. In such cases, the beam is always designed as a rectangular section. Positive beam moments produce bottom steel. In such cases, the beam may be designed as a rectangular or Tbeam. For the design of flexural reinforcement, the beam is first designed as a singly reinforced beam. If the beam section is not adequate, the required compression reinforcement is calculated. In designing the shear reinforcement for a particular beam for a particular set of loading combinations at a particular station resulting from the beam major shear, the steps involve the determination of the factored shear force, the determination of the shear force that can be resisted by concrete, and the determination of the reinforcement steel required to carry the balance. Design of Columns The program designs all concrete frame elements designated as column sections in their Frame Section Properties as columns (see Define menu >Frame Sections command and click the Reinforcement button). In the design of the columns, the program calculates the required longitudinal steel, or if the longitudinal steel is specified, the column stress condition is reported in terms of a column capacity ratio. The capacity ratio is a factor that gives an indication of the stress condition of the column with respect to the capacity of the column. The design procedure for reinforced concrete columns involves the following steps: Generate axial forcebiaxial moment interaction surfaces for all of the different concrete section types of the model. Check the capacity of each column for the factored axial force and bending moments obtained from each load combination at each end of the column. This step is also used to calculate the required reinforcement (if none was specified) that will produce a capacity ratio of 1.0. Design the column shear reinforcement. The shear reinforcement design procedure for columns is very similar to that for beams, except that the effect of the axial force on the concrete shear capacity needs to be considered. See Concrete Frame UBC97 Technical Note 10 Design of Beams Technical Note 13
17 General Design Information Concrete Frame Design Column Design and Concrete Frame ACI Technical Note 20 Column Design for more information. Beam/Column Flexural Capacity Ratios When the ACI or UBC97 code is selected, the program calculates the ratio of the sum of the beam moment capacities to the sum of the column moment capacities at a particular joint for a particular column direction, major or minor. The capacities are calculated with no reinforcing overstrength factor, α, and including ϕ factors. The beam capacities are calculated for reversed situations and the maximum summation obtained is used. The moment capacities of beams that frame into the joint in a direction that is not parallel to the major or minor direction of the column are resolved along the direction that is being investigated and the resolved components are added to the summation. The column capacity summation includes the column above and the column below the joint. For each load combination, the axial force, P u, in each of the columns is calculated from the program analysis load combinations. For each load combination, the moment capacity of each column under the influence of the corresponding axial load P u is then determined separately for the major and minor directions of the column, using the uniaxial column interaction diagram. The moment capacities of the two columns are added to give the capacity summation for the corresponding load combination. The maximum capacity summations obtained from all of the load combinations is used for the beam/column capacity ratio. The beam/column flexural capacity ratios are only reported for Special MomentResisting Frames involving seismic design load combinations. See Beam/Column Flexural Capacity Ratios in Concrete Frame UBC97 Technical Note 12 Joint Design or in Concrete Frame ACI Technical Note 22 Joint Design for more information. Second Order PDelta Effects Typically, design codes require that second order PDelta effects be considered when designing concrete frames. The PDelta effects come from two sources. They are the global lateral translation of the frame and the local deformation of elements within the frame. Technical Note 14 Second Order PDelta Effects
18 Concrete Frame Design General Design Information Original position of frame element shown by vertical line Position of frame element as a result of global lateral translation,, shown by dashed line δ Final deflected position of frame element that includes the global lateral translation,, and the local deformation of the element, δ Figure 1: The Total Second Order PDelta Effects on a Frame Element Caused by Both and δ Consider the frame element shown in Figure 1, which is extracted from a story level of a larger structure. The overall global translation of this frame element is indicated by. The local deformation of the element is shown as δ. The total second order PDelta effects on this frame element are those caused by both and δ. The program has an option to consider PDelta effects in the analysis. Controls for considering this effect are found using the Analyze menu > Set Analysis Options command and then clicking the Set PDelta Parameters button. When you consider PDelta effects in the analysis, the program does a good job of capturing the effect due to the deformation shown in Figure 1, but it does not typically capture the effect of the δ deformation (unless, in the model, the frame element is broken into multiple pieces over its length). In design codes, consideration of the second order PDelta effects is generally achieved by computing the flexural design capacity using a formula similar to that shown in Equation. 1. where, M CAP = am nt + bm lt Eqn. 1 M CAP = Flexural design capacity Second Order PDelta Effects Technical Note 15
19 General Design Information Concrete Frame Design M nt = Required flexural capacity of the member assuming there is no translation of the frame (i.e., associated with the δ deformation in Figure 1) M lt = Required flexural capacity of the member as a result of lateral translation of the frame only (i.e., associated with the deformation in Figure 1) a = Unitless factor multiplying M nt b = Unitless factor multiplying M lt (assumed equal to 1 by the program; see below) When the program performs concrete frame design, it assumes that the factor b is equal to 1 and it uses codespecific formulas to calculate the factor a. That b = 1 assumes that you have considered PDelta effects in the analysis, as previously described. Thus, in general, if you are performing concrete frame design in this program, you should consider PDelta effects in the analysis before running the design. Element Unsupported Lengths The column unsupported lengths are required to account for column slenderness effects. The program automatically determines these unsupported lengths. They can also be overwritten by the user on an elementbyelement basis, if desired, using the Design menu > Concrete Frame Design > View/Revise Overwrites command. There are two unsupported lengths to consider. They are L 33 and L 22, as shown in Figure 2. These are the lengths between support points of the element in the corresponding directions. The length L 33 corresponds to instability about the 33 axis (major axis), and L 22 corresponds to instability about the 22 axis (minor axis). The length L 22 is also used for lateraltorsional buckling caused by major direction bending (i.e., about the 33 axis). In determining the values for L 22 and L 33 of the elements, the program recognizes various aspects of the structure that have an effect on these lengths, such as member connectivity, diaphragm constraints and support points. The program automatically locates the element support points and evaluates the corresponding unsupported length. Technical Note 16 Element Unsupported Lengths
20 Concrete Frame Design General Design Information Figure 2: Major and Minor Axes of Bending It is possible for the unsupported length of a frame element to be evaluated by the program as greater than the corresponding element length. For example, assume a column has a beam framing into it in one direction, but not the other, at a floor level. In this case, the column is assumed to be supported in one direction only at that story level, and its unsupported length in the other direction will exceed the story height. Analysis Sections and Design Sections Analysis sections are those section properties used to analyze the model when you click the Analyze menu > Run Analysis command. The design section is whatever section has most currently been designed and thus designated the current design section. Tip: It is important to understand the difference between analysis sections and design sections. Analysis Sections and Design Sections Technical Note 17
21 General Design Information Concrete Frame Design It is possible for the last used analysis section and the current design section to be different. For example, you may have run your analysis using a W18X35 beam and then found in the design that a W16X31 beam worked. In that case, the last used analysis section is the W18X35 and the current design section is the W16X31. Before you complete the design process, verify that the last used analysis section and the current design section are the same. The Design menu > Concrete Frame Design > Verify Analysis vs Design Section command is useful for this task. The program keeps track of the analysis section and the design section separately. Note the following about analysis and design sections: Assigning a beam a frame section property using the Assign menu > Frame/Line > Frame Section command assigns the section as both the analysis section and the design section. Running an analysis using the Analyze menu > Run Analysis command (or its associated toolbar button) always sets the analysis section to be the same as the current design section. Assigning an auto select list to a frame section using the Assign menu > Frame/Line > Frame Section command initially sets the design section to be the beam with the median weight in the auto select list. Unlocking a model deletes the design results, but it does not delete or change the design section. Using the Design menu > Concrete Frame Design > Select Design Combo command to change a design load combination deletes the design results, but it does not delete or change the design section. Using the Define menu > Load Combinations command to change a design load combination deletes the design results, but it does not delete or change the design section. Using the Options menu > Preferences > Concrete Frame Design command to change any of the composite beam design preferences deletes the design results, but it does not delete or change the design section. Deleting the static nonlinear analysis results also deletes the design results for any load combination that includes static nonlinear forces. Typically, Technical Note 18 Analysis Sections and Design Sections
22 Concrete Frame Design General Design Information static nonlinear analysis and design results are deleted when one of the following actions is taken: Use the Define menu > Frame Nonlinear Hinge Properties command to redefine existing or define new hinges. Use the Define menu > Static Nonlinear/Pushover Cases command to redefine existing or define new static nonlinear load cases. Use the Assign menu > Frame/Line > Frame Nonlinear Hinges command to add or delete hinges. Again, note that these actions delete only results for load combinations that include static nonlinear forces. Analysis Sections and Design Sections Technical Note 19
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24 COMPUTERS AND STRUCTURES, INC., BERKELEY, CALIFORNIA DECEMBER 2001 CONCRETE FRAME DESIGN Technical Note 2 Concrete Frame Design Process This Technical Note describes a basic concrete frame design process using this program. Although the exact steps you follow may vary, the basic design process should be similar to that described herein. Other Technical Notes in the Concrete Frame Design series provide additional information, including the distinction between analysis sections and design sections (see Analysis Sections and Design Sections in Concrete Frame Design Technical Note 1 General Design Information). The concrete frame design postprocessor can design or check concrete columns and can design concrete beams. Important note: A concrete frame element is designed as a beam or a column, depending on how its frame section property was designated when it was defined using the Define menu > Frame Sections command. Note that when using this command, after you have specified that a section has a concrete material property, you can click on the Reinforcement button and specify whether it is a beam or a column. Concrete Frame Design Procedure The following sequence describes a typical concrete frame design process for a new building. Note that although the sequence of steps you follow may vary, the basic process probably will be essentially the same. 1. Use the Options menu > Preferences > Concrete Frame Design command to choose the concrete frame design code and to review other concrete frame design preferences and revise them if necessary. Note that default values are provided for all concrete frame design preferences, so it is unnecessary to define any preferences unless you want to change some of the default values. See Concrete Frame Design ACI UBC97 Technical Notes 6 Preferences and Concrete Frame Design ACI Technical Notes 16 Preferences for more information. Concrete Frame Design Procedure Technical Note 21
25 Concrete Frame Design Process Concrete Frame Design 2. Create the building model. 3. Run the building analysis using the Analyze menu > Run Analysis command. 4. Assign concrete frame overwrites, if needed, using the Design menu > Concrete Frame Design > View/Revise Overwrites command. Note that you must select frame elements before using this command. Also note that default values are provided for all concrete frame design overwrites, so it is unnecessary to define any overwrites unless you want to change some of the default values. Note that the overwrites can be assigned before or after the analysis is run. See Concrete Frame Design UBC97 Technical Note 7 Overwrites and Concrete Frame Design ACI Technical Note 17 Overwrites for more information. 5. To use any design load combinations other than the defaults created by the program for your concrete frame design, click the Design menu > Concrete Frame Design > Select Design Combo command. Note that you must have already created your own design combos by clicking the Define menu > Load Combinations command. See Concrete Frame Design UBC97 Technical Note 8 Design Load Combinations and Concrete Frame Design ACI Technical Note 18 Design Load Combinations for more information. 6. Click the Design menu > Concrete Frame Design > Start Design/Check of Structure command to run the concrete frame design. 7. Review the concrete frame design results by doing one of the following: a. Click the Design menu > Concrete Frame Design > Display Design Info command to display design input and output information on the model. See Concrete Frame Design Technical Note 4 Output Data Plotted Directly on the Model for more information. b. Right click on a frame element while the design results are displayed on it to enter the interactive design mode and interactively design the frame element. Note that while you are in this mode, you can revise overwrites and immediately see the results of the new design. See Concrete Frame Design Technical Note 3 Interactive Concrete Frame Design for more information. Technical Note 22 Concrete Frame Design Procedure
26 Concrete Frame Design Concrete Frame Design Process If design results are not currently displayed (and the design has been run), click the Design menu > Concrete Frame Design > Interactive Concrete Frame Design command and then right click a frame element to enter the interactive design mode for that element. 8. Use the File menu > Print Tables > Concrete Frame Design command to print concrete frame design data. If you select frame elements before using this command, data is printed only for the selected elements. See Concrete Frame Design UBC97 Technical Note 14 Output Details and Concrete Frame Design ACI Technical Note 24 Output Details for more information. 9. Use the Design menu > Concrete Frame Design > Change Design Section command to change the design section properties for selected frame elements. 10. Click the Design menu > Concrete Frame Design > Start Design/Check of Structure command to rerun the concrete frame design with the new section properties. Review the results using the procedures described in Item Rerun the building analysis using the Analyze menu > Run Analysis command. Note that the section properties used for the analysis are the last specified design section properties. 12. Click the Design menu > Concrete Frame Design > Start Design/Check of Structure command to rerun the concrete frame design with the new analysis results and new section properties. Review the results using the procedures described above. 13. Again use the Design menu > Concrete Frame Design > Change Design Section command to change the design section properties for selected frame elements, if necessary. 14. Repeat the processes in steps 10, 11 and 12 as many times as necessary. 15. Rerun the building analysis using the Analyze menu > Run Analysis command. Note that the section properties used for the analysis are the last specified design section properties. Concrete Frame Design Procedure Technical Note 23
27 Concrete Frame Design Process Concrete Frame Design Note: Concrete frame design is an iterative process. Typically, the analysis and design will be rerun multiple times to complete a design. 16. Click the Design menu > Concrete Frame Design > Start Design/Check of Structure command to rerun the concrete frame design with the new section properties. Review the results using the procedures described in Item Click the Design menu > Concrete Frame Design > Verify Analysis vs Design Section command to verify that all of the final design sections are the same as the last used analysis sections. 18. Use the File menu > Print Tables > Concrete Frame Design command to print selected concrete frame design results, if desired. It is important to note that design is an iterative process. The sections used in the original analysis are not typically the same as those obtained at the end of the design process. Always run the building analysis using the final frame section sizes and then run a design check using the forces obtained from that analysis. Use the Design menu > Concrete Frame Design > Verify Analysis vs Design Section command to verify that the design sections are the same as the analysis sections. Technical Note 24 Concrete Frame Design Procedure
28 COMPUTERS AND STRUCTURES, INC., BERKELEY, CALIFORNIA DECEMBER 2001 CONCRETE FRAME DESIGN Technical Note 3 Interactive Concrete Frame Design This Technical Note describes interactive concrete frame design and review, which is a powerful mode that allows the user to review the design results for any concrete frame design and interactively revise the design assumptions and immediately review the revised results. General Note that a design must have been run for the interactive design mode to be available. To run a design, click the Design menu > Concrete Frame Design > Start Design/Check of Structure command. Right click on a frame element while the design results are displayed on it to enter the interactive design mode and interactively design the element in the Concrete Design Information form. If design results are not currently displayed (and a design has been run), click the Design menu > Concrete Frame Design > Interactive Concrete Frame Design command and then right click a frame element to enter the interactive design mode for that element. Important note: A concrete frame element is designed as a beam or a column, depending on how its frame section property was designated when it was defined using the Define menu > Frame Sections command and the Reinforcement button, which is only available if it is a concrete section. Concrete Design Information Form Table 1 describe the features that are included in the Concrete Design Information form. General Technical Note 31
29 Interactive Concrete Frame Design Concrete Frame Design Table 1 Concrete Design Information Form Item Story Beam Column Section Name DESCRIPTION This is the story level ID associated with the frame element. This is the label associated with a frame element that has been assigned a concrete frame section property that is designated as a beam. See the important note previously in this Technical Note for more information. This is the label associated with a frame element that has been assigned a concrete frame section property that is designated as a column. See the important note previously in this Technical Note for more information. This is the label associated with a frame element that has been assigned a concrete frame section property. Reinforcement Information The reinforcement information table on the Concrete Design Information form shows the output information obtained for each design load combination at each output station along the frame element. For columns that are designed by this program, the item with the largest required amount of longitudinal reinforcing is initially highlighted. For columns that are checked by this program, the item with the largest capacity ratio is initially highlighted. For beams, the item with the largest required amount of bottom steel is initially highlighted. Following are the possible headings in the table: Combo ID Station location Longitudinal reinforcement Capacity ratio This is the name of the design load combination considered. This is the location of the station considered, measured from the iend of the frame element. This item applies to columns only. It also only applies to columns for which the program designs the longitudinal reinforcing. It is the total required area of longitudinal reinforcing steel. This item applies to columns only. It also only applies to columns for which you have specified the location and size of reinforcing bars and thus the program checks the design. This item is the capacity ratio. Technical Note 32 Table 1 Concrete Design Information Form
30 Concrete Frame Design Interactive Concrete Frame Design Table 1 Concrete Design Information Form Item Major shear reinforcement Minor shear reinforcement Top steel Bottom steel Shear steel Overwrites Button DESCRIPTION The capacity ratio is determined by first extending a line from the origin of the PMM interaction surface to the point representing the P, M2 and M3 values for the designated load combination. Assume the length of this first line is designated L1. Next, a second line is extended from the origin of the PMM interaction surface through the point representing the P, M2 and M3 values for the designated load combination until it intersects the interaction surface. Assume the length of this line from the origin to the interaction surface is designated L2. The capacity ratio is equal to L1/L2. This item applies to columns only. It is the total required area of shear reinforcing per unit length for shear acting in the column major direction. This item applies to columns only. It is the total required area of shear reinforcing per unit length for shear acting in the column minor direction. This item applies to beams only. It is the total required area of longitudinal top steel at the specified station. This item applies to beams only. It is the total required area of longitudinal bottom steel at the specified station. This item applies to beams only. It is the total required area of shear reinforcing per unit length at the specified station for loads acting in the local 2axis direction of the beam. Click this button to access and make revisions to the concrete frame overwrites and then immediately see the new design results. If you modify some overwrites in this mode and you exit both the Concrete Frame Design Overwrites form and the Concrete Design Information form by clicking their respective OK buttons, the changes to the overwrites are saved permanently. When you exit the Concrete Frame Design Overwrites form by clicking the OK button the changes are temporarily saved. If you then exit the Concrete Design Information form by clicking the Cancel button the changes you made to the concrete frame overwrites are considered temporary only and are not permanently saved. Permanent saving of the overwrites does not actually occur until you click the OK button in the Concrete Design Information form as well as the Concrete Frame Design Overwrites form. Table 1 Concrete Design Information Form Technical Note 33
31 Interactive Concrete Frame Design Concrete Frame Design Table 1 Concrete Design Information Form Item Details Button Interaction Button DESCRIPTION Clicking this button displays design details for the frame element. Print this information by selecting Print from the File menu that appears at the top of the window displaying the design details. Clicking this button displays the biaxial interaction curve for the concrete section at the location in the element that is highlighted in the table. Technical Note 34 Table 1 Concrete Design Information Form
32 COMPUTERS AND STRUCTURES, INC., BERKELEY, CALIFORNIA DECEMBER 2001 CONCRETE FRAME DESIGN Technical Note 4 Output Data Plotted Directly on the Model This Technical Note describes the input and output data that can be plotted directly on the model. Overview Use the Design menu > Concrete Frame Design > Display Design Info command to display onscreen output plotted directly on the program model. If desired, the screen graphics can then be printed using the File menu > Print Graphics command. The onscreen display data presents input and output data. Using the Print Design Tables Form To print the concrete frame input summary directly to a printer, use the File menu > Print Tables > Concrete Frame Design command and click the check box on the Print Design Tables form. Click the OK button to send the print to your printer. Click the Cancel button rather than the OK button to cancel the print. Use the File menu > Print Setup command and the Setup>> button to change printers, if necessary. To print the concrete frame input summary to a file, click the Print to File check box on the Print Design Tables form. Click the Filename>> button to change the path or filename. Use the appropriate file extension for the desired format (e.g.,.txt,.xls,.doc). Click the OK buttons on the Open File for Printing Tables form and the Print Design Tables form to complete the request. Note: The File menu > Display Input/Output Text Files command is useful for displaying output that is printed to a text file. The Append check box allows you to add data to an existing file. The path and filename of the current file is displayed in the box near the bottom of the Print Design Tables form. Data will be added to this file. Or use the Filename Overview Technical Note 41
33 Output Data Plotted Directly on the Model Concrete Frame Design button to locate another file, and when the Open File for Printing Tables caution box appears, click Yes to replace the existing file. If you select a specific concrete frame element(s) before using the File menu > Print Tables > concrete Frame Design command, the Selection Only check box will be checked. The print will be for the selected steel frame element(s) only. Design Input The following types of data can be displayed directly on the model by selecting the data type (shown in bold type) from the dropdown list on the Display Design Results form. Display this form by selecting he Design menu > Concrete Frame Design > Display Design Info command. Design Sections Design Type Live Load Red Factors Unbraced L_Ratios Eff Length KFactors Cm Factors DNS Factors DS Factors Each of these items is described in the codespecific Concrete Frame Design UBC97 Technical Note 13 Input Data and Concrete Frame Design ACI Technical Note 23 Input Data. Design Output The following types of data can be displayed directly on the model by selecting the data type (shown in bold type) from the dropdown list on the Display Design Results form. Display this form by selecting he Design menu > Concrete Frame Design > Display Design Info command. Technical Note 42 Design Input
34 Concrete Frame Design Output Data Plotted Directly on the Model Longitudinal Reinforcing Shear Reinforcing Column Capacity Ratios Joint Shear Capacity Ratios Beam/Column Capacity Ratios Each of these items is described in the codespecific Concrete Frame Design ACI Technical Note 24 Output Details and Concrete Frame Design UBC97 Technical Note 14 Output Details. Design Output Technical Note 43
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36 COMPUTERS AND STRUCTURES, INC., BERKELEY, CALIFORNIA DECEMBER 2001 CONCRETE FRAME DESIGN UBC97 Technical Note 5 General and Notation Introduction to the UBC97 Series of Technical Notes The Concrete Frame Design UBC97 series of Technical Notes describes in detail the various aspects of the concrete design procedure that is used by this program when the user selects the UBC97 Design Code (ICBO 1997). The various notations used in this series are listed herein. The design is based on userspecified loading combinations. The program provides a set of default load combinations that should satisfy requirements for the design of most building type structures. See Concrete Frame Design UBC97 Technical Note 8 Design Load Combinations for more information. When using the UBC 97 option, a frame is assigned to one of the following five Seismic Zones (UBC 2213, 2214): Zone 0 Zone 1 Zone 2 Zone 3 Zone 4 By default the Seismic Zone is taken as Zone 4 in the program. However, the Seismic Zone can be overwritten in the Preference form to change the default. See Concrete Frame Design UBC97 Technical Note 6 Preferences for more information. When using the UBC 97 option, the following Framing Systems are recognized and designed according to the UBC design provisions (UBC 1627, 1921): Ordinary MomentResisting Frame (OMF) General and Notation Technical Note 51
37 General and Notation Concrete Frame Design UBC97 Intermediate MomentResisting Frame (IMRF) Special MomentResisting Frame (SMRF) The Ordinary MomentResisting Frame (OMF) is appropriate in minimal seismic risk areas, especially in Seismic Zones 0 and 1. The Intermediate MomentResisting Frame (IMRF) is appropriate in moderate seismic risk areas, specially in Seismic Zone 2. The Special MomentResisting Frame (SMRF) is appropriate in high seismic risk areas, specially in Seismic Zones 3 and 4. The UBC seismic design provisions are considered in the program. The details of the design criteria used for the different framing systems are described in Concrete Frame Design UBC97 Technical Note 9 Strength Reduction Factors, Concrete Frame Design UBC97 Technical Note 10 Column Design, Concrete Frame Design UBC97 Technical Note 11 Beam Design, and Concrete Frame Design UBC97 Technical Note 12 Joint Design. By default the frame type is taken in the program as OMRF in Seismic Zone 0 and 1, as IMRF in Seismic Zone 2, and as SMRF in Seismic Zone 3 and 4. However, the frame type can be overwritten in the Overwrites form on a memberbymember basis. See Concrete Frame Design UBC97 Technical Note 7 Overwrites for more information. If any member is assigned with a frame type, the change of the Seismic Zone in the Preferences will not modify the frame type of an individual member that has been assigned a frame type. The program also provides input and output data summaries, which are described in Concrete Frame Design UBC97 Technical Note 13 Input Data and Concrete Frame Design UBC97 Technical Note 14 Output Details. English as well as SI and MKS metric units can be used for input. The code is based on InchPoundSecond units. For simplicity, all equations and descriptions presented in this Technical Note correspond to InchPoundSecond units unless otherwise noted. Notation A cv Area of concrete used to determine shear stress, sqin A g A s Gross area of concrete, sqin Area of tension reinforcement, sqin Technical Note 52 General and Notation
38 Concrete Frame Design UBC97 General and Notation ' As Area of compression reinforcement, sqin A s(required) A st A v C m D' E c E s I g I se L M 1 M 2 M c M ns M s M u M ux M uy P b Area of steel required for tension reinforcement, sqin Total area of column longitudinal reinforcement, sqin Area of shear reinforcement, sqin Coefficient, dependent upon column curvature, used to calculate moment magnification factor Diameter of hoop, in Modulus of elasticity of concrete, psi Modulus of elasticity of reinforcement, assumed as 29,000,000 psi (UBC ) Moment of inertia of gross concrete section about centroidal axis, neglecting reinforcement, in 4 Moment of inertia of reinforcement about centroidal axis of member cross section, in 4 Clear unsupported length, in Smaller factored end moment in a column, lbin Larger factored end moment in a column, lbin Factored moment to be used in design, lbin Nonsway component of factored end moment, lbin Sway component of factored end moment, lbin Factored moment at section, lbin Factored moment at section about Xaxis, lbin Factored moment at section about Yaxis, lbin Axial load capacity at balanced strain conditions, lb General and Notation Technical Note 53
39 General and Notation Concrete Frame Design UBC97 P c P max P 0 P u V c V E V D+L V u V p a a b b b f b w c c b d d' d s ' fc Critical buckling strength of column, lb Maximum axial load strength allowed, lb Acial load capacity at zero eccentricity, lb Factored axial load at section, lb Shear resisted by concrete, lb Shear force caused by earthquake loads, lb Shear force from span loading, lb Factored shear force at a section, lb Shear force computed from probable moment capacity, lb Depth of compression block, in Depth of compression block at balanced condition, in Width of member, in Effective width of flange (TBeam section), in Width of web (TBeam section), in Depth to neutral axis, in Depth to neutral axis at balanced conditions, in Distance from compression face to tension reinforcement, in Concrete cover to center of reinforcing, in Thickness of slab (TBeam section), in Specified compressive strength of concrete, psi f y Specified yield strength of flexural reinforcement, psi f y 80,000 psi (UBC ) Technical Note 54 General and Notation
40 Concrete Frame Design UBC97 General and Notation f ys h k r α β 1 β d δ s δ ns ε c ε s ϕ Specified yield strength of flexural reinforcement, psi Dimension of column, in Effective length factor Radius of gyration of column section, in Reinforcing steel overstrength factor Factor for obtaining depth of compression block in concrete Absolute value of ratio of maximum factored axial dead load to maximum factored axial total load Moment magnification factor for sway moments Moment magnification factor for nonsway moments Strain in concrete Strain in reinforcing steel Strength reduction factor General and Notation Technical Note 55
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42 COMPUTERS AND STRUCTURES, INC., BERKELEY, CALIFORNIA DECEMBER 2001 CONCRETE FRAME DESIGN UBC97 Technical Note 6 Preferences This Technical Note describes the items in the Preferences form. General The concrete frame design preferences in this program are basic assignments that apply to all concrete frame elements. Use the Options menu > Preferences > Concrete Frame Design command to access the Preferences form where you can view and revise the concrete frame design preferences. Default values are provided for all concrete frame design preference items. Thus, it is not required that you specify or change any of the preferences. You should, however, at least review the default values for the preference items to make sure they are acceptable to you. Using the Preferences Form To view preferences, select the Options menu > Preferences > Concrete Frame Design. The Preferences form will display. The preference options are displayed in a twocolumn spreadsheet. The left column of the spreadsheet displays the preference item name. The right column of the spreadsheet displays the preference item value. To change a preference item, left click the desired preference item in either the left or right column of the spreadsheet. This activates a dropdown box or highlights the current preference value. If the dropdown box appears, select a new value. If the cell is highlighted, type in the desired value. The preference value will update accordingly. You cannot overwrite values in the dropdown boxes. When you have finished making changes to the concrete frame preferences, click the OK button to close the form. You must click the OK button for the changes to be accepted by the program. If you click the Cancel button to exit General Technical Note 61
43 Preferences Concrete Frame Design UBC97 the form, any changes made to the preferences are ignored and the form is closed. Preferences For purposes of explanation in this Technical Note, the preference items are presented in Table 1. The column headings in the table are described as follows: Item: The name of the preference item as it appears in the cells at the left side of the Preferences form. Possible Values: The possible values that the associated preference item can have. Default Value: The builtin default value that the program assumes for the associated preference item. Description: A description of the associated preference item. Table 1: Concrete Frame Preferences Item Possible Values Default Value Description Design Code Any code in the program UBC97 Design code used for design of concrete frame elements. Phi Bending Tension Phi Compression Tied Phi Compression Spiral >0 0.9 Unitless strength reduction factor per UBC >0 0.7 Unitless strength reduction factor per UBC > Unitless strength reduction factor per UBC Phi Shear > Unitless strength reduction factor per UBC Number Interaction Curves Number of equally spaced interaction curves used to create a full 360degree interaction surface (this item should be a multiple of four). We recommend that you use 24 for this item. Technical Note 62 Preferences
44 Concrete Frame Design UBC97 Preferences Table 1: Concrete Frame Preferences Item Number Interaction Points Time History Design Possible Values Any odd value 1.0 Envelopes or StepbyStep Default Value Description 11 Number of points used for defining a single curve in a concrete frame interaction surface (this item should be odd). Envelopes Toggle for design load combinations that include a time history designed for the envelope of the time history, or designed stepbystep for the entire time history. If a single design load combination has more than one time history case in it, that design load combination is designed for the envelopes of the time histories, regardless of what is specified here. Preferences Technical Note 63
45
46 COMPUTERS AND STRUCTURES, INC., BERKELEY, CALIFORNIA DECEMBER 2001 CONCRETE FRAME DESIGN UBC97 Technical Note 7 Overwrites General The concrete frame design overwrites are basic assignments that apply only to those elements to which they are assigned. This Technical Note describes concrete frame design overwrites for UBC97. To access the overwrites, select an element and click the Design menu > Concrete Frame Design > View/Revise Overwrites command. Default values are provided for all overwrite items. Thus, you do not need to specify or change any of the overwrites. However, at least review the default values for the overwrite items to make sure they are acceptable. When changes are made to overwrite items, the program applies the changes only to the elements to which they are specifically assigned; that is, to the elements that are selected when the overwrites are changed. Overwrites For explanation purposes in this Technical Note, the overwrites are presented in Table 1. The column headings in the table are described as follows. Item: The name of the overwrite item as it appears in the program. To save space in the formes, these names are generally short. Possible Values: The possible values that the associated overwrite item can have. Default Value: The default value that the program assumes for the associated overwrite item. Description: A description of the associated overwrite item. An explanation of how to change an overwrite is provided at the end of this Technical Note. Overwrites Technical Note 71
47 Overwrites Concrete Frame Design UBC97 Table 1 Concrete Frame Design Overwrites Item Possible Values Default Value Description Element Section Element Type Live Load Reduction Factor Horizontal Earthquake Factor Unbraced Length Ratio (Major) Unbraced Length Ratio (Minor) Effective Length Factor (K Major) Effective Length Factor (K Minor) Moment Coefficient (Cm Major) Moment Coefficient (Cm Minor) NonSway Moment Factor (Dns Major) Sway Special, Sway Intermediate, Sway Ordinary NonSway >0 1.0 >0 1.0 >0 1.0 >0 1.0 >0 1.0 >0 1.0 >0 1.0 >0 1.0 >0 1.0 Sway Special Frame type; see UBC to Used to reduce the live load contribution to the factored loading See UBC See UBC See UBC relates actual moment diagram to an equivalent uniform moment diagram. 1 See UBC relates actual moment diagram to an equivalent uniform moment diagram. 1 See UBC Technical Note 72 Overwrites
48 Concrete Frame Design UBC97 Overwrites Table 1 Concrete Frame Design Overwrites Item Possible Values Default Value Description NonSway Moment Factor (Dns Minor) Sway Moment Factor (Ds Major) Sway Moment Factor (Ds Minor) 1 See UBC See UBC See UBC Making Changes in the Overwrites Form To access the concrete frame overwrites, select an element and click the Design menu > Concrete Frame Design > View/Revise Overwrites command. The overwrites are displayed in the form with a column of check boxes and a twocolumn spreadsheet. The left column of the spreadsheet contains the name of the overwrite item. The right column of the spreadsheet contains the overwrites values. Initially, the check boxes in the Concrete Frame Design Overwrites form are all unchecked and all of the cells in the spreadsheet have a gray background to indicate that they are inactive and the items in the cells cannot be changed. The names of the overwrite items are displayed in the first column of the spreadsheet. The values of the overwrite items are visible in the second column of the spreadsheet if only one element was selected before the overwrites form was accessed. If multiple elements were selected, no values show for the overwrite items in the second column of the spreadsheet. After selecting one or multiple elements, check the box to the left of an overwrite item to change it. Then left click in either column of the spreadsheet to activate a dropdown box or highlight the contents in the cell in the right column of the spreadsheet. If the dropdown box appears, select a value from Overwrites Technical Note 73
49 Overwrites Concrete Frame Design UBC97 the box. If the cell contents is highlighted, type in the desired value. The overwrite will reflect the change. You cannot change the values of the dropdown boxes. When changes to the overwrites have been completed, click the OK button to close the form. The program then changes all of the overwrite items whose associated check boxes are checked for the selected members. You must click the OK button for the changes to be accepted by the program. If you click the Cancel button to exit the form, any changes made to the overwrites are ignored and the form is closed. Resetting Concrete Frame Overwrites to Default Values Use the Design menu > Concrete Frame Design > Reset All Overwrites command to reset all of the steel frame overwrites. All current design results will be deleted when this command is executed. Important note about resetting overwrites: The program defaults for the overwrite items are built into the program. The concrete frame overwrite values that were in a.edb file that you used to initialize your model may be different from the builtin program default values. When you reset overwrites, the program resets the overwrite values to its builtin values, not to the values that were in the.edb file used to initialize the model. Technical Note 74 Overwrites
50 COMPUTERS AND STRUCTURES, INC., BERKELEY, CALIFORNIA DECEMBER 2001 CONCRETE FRAME DESIGN UBC97 Technical Note 8 Design Load Combinations The design load combinations are the various combinations of the prescribed load cases for which the structure needs to be checked. For the UBC 97 code, if a structure is subjected to dead load (DL) and live load (LL) only, the stress check may need only one load combination, namely 1.4 DL LL (UBC ). However, in addition to the dead and live loads, if the structure is subjected to wind (WL) and earthquake (EL) loads, and considering that wind and earthquake forces are reversible, the following load combinations may need to be considered (UBC ). 1.4 DL (UBC ) 1.4 DL LL (UBC ) 0.9 DL ± 1.3 WL (UBC ) 0.75 (1.4 DL LL ± 1.7 WL) (UBC ) 0.9 DL ± 1.0 EL (UBC , ) 1.2 DL LL ± 1.0 EL) (UBC , ) These are also the default design load combinations in the program whenever the UBC97 code is used. Live load reduction factors can be applied to the member forces of the live load condition on an elementbyelement basis to reduce the contribution of the live load to the factored loading. See Concrete Frame Design UBC97 Technical Note 7 Overwrites for more information. Design Load Combinations Technical Note 81
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52 COMPUTERS AND STRUCTURES, INC., BERKELEY, CALIFORNIA DECEMBER 2001 CONCRETE FRAME DESIGN UBC97 Technical Note 9 Strength Reduction Factors The strength reduction factors, ϕ, are applied on the nominal strength to obtain the design strength provided by a member. The ϕ factors for flexure, axial force, shear, and torsion are as follows: ϕ = 0.90 for flexure (UBC ) ϕ = 0.90 for axial tension (UBC ) ϕ = 0.90 for axial tension and flexure (UBC ) ϕ = 0.75 for axial compression, and axial compression and flexure (spirally reinforced column) (UBC ) ϕ = 0.70 for axial compression, and axial compression and flexure (tied column) (UBC ) ϕ = 0.85 for shear and torsion (nonseismic design) (UBC ) ϕ = 0.60 for shear and torsion (UBC ) Strength Reduction Factors Technical Note 91
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54 COMPUTERS AND STRUCTURES, INC., BERKELEY, CALIFORNIA DECEMBER 2001 CONCRETE FRAME DESIGN UBC97 Technical Note 10 Column Design This Technical Note describes how the program checks column capacity or designs reinforced concrete columns when the UBC97 code is selected. Overview The program can be used to check column capacity or to design columns. If you define the geometry of the reinforcing bar configuration of each concrete column section, the program will check the column capacity. Alternatively, the program can calculate the amount of reinforcing required to design the column. The design procedure for the reinforced concrete columns of the structure involves the following steps: Generate axial force/biaxial moment interaction surfaces for all of the different concrete section types of the model. A typical biaxial interaction surface is shown in Figure 1. When the steel is undefined, the program generates the interaction surfaces for the range of allowable reinforcement 1 to 8 percent for Ordinary and Intermediate moment resisting frames (UBC ) and 1 to 6 percent for Special moment resisting frames (UBC ). Calculate the capacity ratio or the required reinforcing area for the factored axial force and biaxial (or uniaxial) bending moments obtained from each loading combination at each station of the column. The target capacity ratio is taken as 1 when calculating the required reinforcing area. Design the column shear reinforcement. The following four subsections describe in detail the algorithms associated with this process. Overview Technical Note 101
55 Column Design Concrete Frame Design UBC97 Figure 1 A Typical Column Interaction Surface Generation of Biaxial Interaction Surfaces The column capacity interaction volume is numerically described by a series of discrete points that are generated on the threedimensional interaction failure surface. In addition to axial compression and biaxial bending, the formulation allows for axial tension and biaxial bending considerations. A typical interaction diagram is shown in Figure 1. Technical Note 102 Generation of Biaxial Interaction Surfaces
56 Concrete Frame Design UBC97 Column Design The coordinates of these points are determined by rotating a plane of linear strain in three dimensions on the section of the column. See Figure 2. The linear strain diagram limits the maximum concrete strain, ε c, at the extremity of the section, to (UBC ). The formulation is based consistently upon the general principles of ultimate strength design (UBC ), and allows for any doubly symmetric rectangular, square, or circular column section. The stress in the steel is given by the product of the steel strain and the steel modulus of elasticity, ε s E s, and is limited to the yield stress of the steel, f y (UBC ). The area associated with each reinforcing bar is assumed to be placed at the actual location of the center of the bar and the algorithm does not assume any further simplifications with respect to distributing the area of steel over the cross section of the column, such as an equivalent steel tube or cylinder. See Figure 3. ' c The concrete compression stress block is assumed to be rectangular, with a stress value of 0.85 f (UBC ). See Figure 3. The interaction algorithm provides correction to account for the concrete area that is displaced by the reinforcement in the compression zone. The effects of the strength reduction factor, ϕ, are included in the generation of the interaction surfaces. The maximum compressive axial load is limited to ϕp n(max), where ϕp n(max) = 0.85ϕ[0.85 f c ' (A g A st )+f y A st ] (spiral) (UBC ) ϕp n(max) = 0.85ϕ[0.85 f c ' (A g A st )+f y A st ] (tied) (UBC ) ϕ = 0.70 for tied columns (UBC ) ϕ = 0.75 for spirally reinforced columns (UBC ) The value of ϕ used in the interaction diagram varies from ϕ min to 0.9 based on the axial load. For low values of axial load, ϕ is increased linearly from ϕ min to 0.9 as the nominal capacity ϕp n decreases from the smaller of ϕp b or 0.1 f ' c A g to zero, where P b is the axial force at the balanced condition. In cases involving axial tension, ϕ is always 0.9 (UBC ). Generation of Biaxial Interaction Surfaces Technical Note 103
57 Column Design Concrete Frame Design UBC97 Figure 2 Idealized Strain Distribution for Generation of Interaction Surfaces Technical Note 104 Generation of Biaxial Interaction Surfaces
58 Concrete Frame Design UBC97 Column Design Figure 3 Idealization of Stress and Strain Distribution in a Column Section Calculate Column Capacity Ratio The column capacity ratio is calculated for each loading combination at each output station of each column. The following steps are involved in calculating the capacity ratio of a particular column for a particular loading combination at a particular location: Determine the factored moments and forces from the analysis load cases and the specified load combination factors to give P u, M ux, and M uy. Determine the moment magnification factors for the column moments. Apply the moment magnification factors to the factored moments. Determine whether the point, defined by the resulting axial load and biaxial moment set, lies within the interaction volume. The factored moments and corresponding magnification factors depend on the identification of the individual column as either sway or nonsway. Calculate Column Capacity Ratio Technical Note 105
59 Column Design Concrete Frame Design UBC97 The following three sections describe in detail the algorithms associated with this process. Determine Factored Moments and Forces The factored loads for a particular load combination are obtained by applying the corresponding load factors to all the load cases, giving P u, M ux, and M uy. The factored moments are further increased for nonsway columns, if required, to obtain minimum eccentricities of ( h) inches, where h is the dimension of the column in the corresponding direction (UBC ). Determine Moment Magnification Factors The moment magnification factors are calculated separately for sway (overall stability effect), δ s, and for nonsway (individual column stability effect), δ ns. Also the moment magnification factors in the major and minor directions are in general different. The program assumes that it performs a Pdelta analysis and, therefore, moment magnification factors for moments causing sidesway are taken as unity (UBC ). For the Pdelta analysis, the load should correspond to a load combination of 0.75 (1.4 dead load live load)/ϕ if wind load governs, or (1.2 dead load live load)/ϕ if seismic load governs, where ϕ is the understrength factor for stability, which is taken as 0.75 (UBC ). See also White and Hajjar (1991). The moment obtained from analysis is separated into two components: the sway (M s ) and the nonsway (M s ) components. The nonsway components which are identified by ns subscripts are predominantly caused by gravity load. The sway components are identified by the s subscripts. The sway moments are predominantly caused by lateral loads, and are related to the cause of sidesway. For individual columns or columnmembers in a floor, the magnified moments about two axes at any station of a column can be obtained as M = M ns + δ s M s. (UBC ) The factor δ s is the moment magnification factor for moments causing side sway. The moment magnification factors for sway moments, δ s, is taken as 1 because the component moments M s and M ns are obtained from a second order elastic (Pdelta) analysis. Technical Note 106 Calculate Column Capacity Ratio
60 Concrete Frame Design UBC97 Column Design The computed moments are further amplified for individual column stability effect (UBC , ) by the nonsway moment magnification factor, δ ns, as follows: M c = δ ns M 2, where (UBC ) M c is the factored moment to be used in design, and M 2 is the larger factored and amplified end moment. The nonsway moment magnification factor, δ ns, associated with the major or minor direction of the column is given by (UBC ) δ ns = Cm Pu P c 1.0, where (UBC ) P c = π 2 EI ( kl u ) 2, (UBC ) k is conservatively taken as 1; however, the program allows the user to override this value. EI is associated with a particular column direction given by: EI = 0.4E c I g 1 + β d, (UBC ) maximum factored axial dead load β d = maximum factored axial total load and (UBC ) Ma C m = (UBC ) M b M a and M b are the moments at the ends of the column, and M b is numerically larger than M a. M a / M b is positive for single curvature bending and negative for double curvature bending. The above expression of C m is valid if there is no transverse load applied between the supports. If transverse load is present on the span, or the length is overwritten, C m = 1. C m can be overwritten by the user on an elementbyelement basis. Calculate Column Capacity Ratio Technical Note 107
61 Column Design Concrete Frame Design UBC97 The magnification factor, δ ns, must be a positive number and greater than 1. Therefore, P u must be less than 0.75P c. If P u is found to be greater than or equal to 0.75P c, a failure condition is declared. The above calculations use the unsupported length of the column. The two unsupported lengths are l 22 and l 33, corresponding to instability in the minor and major directions of the element, respectively. See Figure 4. These are the lengths between the support points of the element in the corresponding directions. Figure 4 Axes of Bending and Unsupported Length If the program assumptions are not satisfactory for a particular member, the user can explicitly specify values of δ s and δ ns. Determine Capacity Ratio The program calculates a capacity ratio as a measure of the stress condition of the column. The capacity ratio is basically a factor that gives an indication Technical Note 108 Calculate Column Capacity Ratio
62 Concrete Frame Design UBC97 Column Design of the stress condition of the column with respect to the capacity of the column. Before entering the interaction diagram to check the column capacity, the moment magnification factors are applied to the factored loads to obtain P u, M ux, and M uy. The point (P u, M ux, M uy.) is then placed in the interaction space shown as point L in Figure 5. If the point lies within the interaction volume, the column capacity is adequate; however, if the point lies outside the interaction volume, the column is overstressed. Figure 5 Geometric Representation of Column Capacity Ratios This capacity ratio is achieved by plotting the point L and determining the location of point C. The point C is defined as the point where the line OL (if extended outwards) will intersect the failure surface. This point is determined by threedimensional linear interpolation between the points that define the Calculate Column Capacity Ratio Technical Note 109
63 Column Design Concrete Frame Design UBC97 failure surface. See Figure 5. The capacity ratio, CR, is given by the ratio OL. OC If OL = OC (or CR=1), the point lies on the interaction surface and the column is stressed to capacity. If OL < OC (or CR<1), the point lies within the interaction volume and the column capacity is adequate. If OL > OC (or CR>1), the point lies outside the interaction volume and the column is overstressed. The maximum of all the values of CR calculated from each load combination is reported for each check station of the column, along with the controlling P u, M ux, and M uy set and associated load combination number. Required Reinforcing Area If the reinforcing area is not defined, the program computes the reinforcement that will give a column capacity ratio of one, calculated as described in the previous section entitled "Calculate Column Capacity Ratio." Design Column Shear Reinforcement The shear reinforcement is designed for each loading combination in the major and minor directions of the column. The following steps are involved in designing the shear reinforcing for a particular column for a particular load combination caused by shear forces in a particular direction: Determine the factored forces acting on the section, P u and V u. Note that P u is needed for the calculation of V c. Determine the shear force, V c, that can be resisted by concrete alone. Calculate the reinforcement steel required to carry the balance. For Special and Intermediate moment resisting frames (Ductile frames), the shear design of the columns is also based on the probable and nominal moment capacities of the members, respectively, in addition to the factored Technical Note Required Reinforcing Area
64 Concrete Frame Design UBC97 Column Design moments. Effects of the axial forces on the column moment capacities are included in the formulation. The following three sections describe in detail the algorithms associated with this process. Determine Section Forces In the design of the column shear reinforcement of an Ordinary moment resisting concrete frame, the forces for a particular load combination, namely, the column axial force, P u, and the column shear force, V u, in a particular direction are obtained by factoring the program analysis load cases with the corresponding load combination factors. In the shear design of Special moment resisting frames (i.e., seismic design) the column is checked for capacityshear in addition to the requirement for the Ordinary moment resisting frames. The capacityshear force in a column, V p, in a particular direction is calculated from the probable moment capacities of the column associated with the factored axial force acting on the column. For each load combination, the factored axial load, P u, is calculated. Then, + the positive and negative moment capacities, M u and M u, of the column in a particular direction under the influence of the axial force P u is calculated using the uniaxial interaction diagram in the corresponding direction. The design shear force, V u, is then given by (UBC ) V u = V p + V D+L (UBC ) where, V p is the capacityshear force obtained by applying the calculated probable ultimate moment capacities at the two ends of the column acting in two opposite directions. Therefore, V p is the maximum of V and V, where P 1 P 2 V P 1 = M I + L M + J, and V P 2 = M + I + L M J, where Design Column Shear Reinforcement Technical Note 1011
65 Column Design Concrete Frame Design UBC97 + I M I M, + J M J M,, = Positive and negative moment capacities at end I of the column using a steel yield stress value of αf y and no ϕ factors (ϕ = 1.0),, = Positive and negative moment capacities at end J of the column using a steel yield stress value of αf y and no ϕ factors (ϕ = 1.0), and L = Clear span of column. For Special moment resisting frames, α is taken as 1.25 (UBC ). V D+L is the contribution of shear force from the inspan distribution of gravity loads. For most of the columns, it is zero. For Intermediate moment resisting frames, the shear capacity of the column is also checked for the capacityshear based on the nominal moment capacities at the ends and the factored gravity loads, in addition to the check required for Ordinary moment resisting frames. The design shear force is taken to be the minimum of that based on the nominal (ϕ = 1.0) moment capacity and factored shear force. The procedure for calculating nominal moment capacity is the same as that for computing the probable moment capacity for special moment resisting frames, except that α is taken equal to 1 rather than 1.25 (UBC , ). The factored shear forces are based on the specified load factors, except the earthquake load factors are doubled (UBC ). Determine Concrete Shear Capacity Given the design force set P u and V u, the shear force carried by the concrete, V c, is calculated as follows: If the column is subjected to axial compression, i.e., P u is positive, V c = 2 f ' c Pu 1 + 2,000A g A cv, (UBC ) where, ' fc 100 psi, and (UBC ) Technical Note Design Column Shear Reinforcement
66 Concrete Frame Design UBC97 Column Design V c 3.5 ' fc P u 1 + A cv. (UBC ) A 500 g Pu The term must have psi units. A cv is the effective shear area which is Ag shown shaded in Figure 6. For circular columns, A cv is not taken to be greater than 0.8 times the gross area (UBC ). Figure 6 Shear Stress Area, A cv Design Column Shear Reinforcement Technical Note 1013
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