REVISION 1: October, 2010
ABOUT THE MANUAL This manual is written in simple and straight language. The main focus was not to tell how to make model, rather how to check the model and to understand its various useful options for making a sound and stable structural error free model. This manual is neither designed for fresh users nor expert users of ETABS, rather it is to clear some doubts of those users who are still beginners of ETABS but know well its interface, but I hope, advance users of ETABS will also find new things to learn from this manual. Due to time constraints and knowledge, I may not be able to address the whole issues. Please send me your suggestions for improvement. Anyone interested to share his/her knowledge or willing to contribute either totally a new section about ETABS or within this section is encouraged. Advance users can also submit their content about advance analysis options like P DELTA, Dynamic, Earthquake/Wind, Response Spectrum, Time History analysis etc. You can also suggest or submit any examples and hand calculations. For further details: My Structural Forum: Email: http://www.coolestbliss363.blogspot.com/ coolestbliss@gmail.com CIVILEA User Account: http://forum.civilea.com/member.php?action=profile&uid=10336 MS WORD format to submit contents: http://www.4shared.com/file/sxauywbg/format.html
DEDICATED TO MY PARENTS, TEACHERS, FRIENDS, & CIVILEA USERS
Table of Contents I STABILITY ERRORS... 1 II CONFIGURATION ERRORS... 4 III TIPS/TRICKS... 12
I STABILITY ERRORS 1 LAST ANALYSIS RUN LOG After running your model, please always check your analysis log for global errors. These errors should be of magnitude 10e 10 or smaller. If you find larger errors, please unlock your model and check your model. 2 CHECK MODEL Another very important tool in ETABS for model checking is found under Analyze menu and that is Check Model option. These checks are: There is also an option to enter tolerance value for these checks to be performed. By default the value is 0.001m or 1mm. You can go as far as 0.025m or 25mm, if you are having real trouble in your model. Line, point and area checks verify for overlaps and any area or line not connected to points or nodes. Sometimes area overlap problem arises when you model has curves. To save time and to work with each check efficiently, you can check your model for each check separately. i Line checks ii Point checks iii Area checks iv Loading and mesh checks Page 1 of 28
After pressing OK button, ETABS will start to check your model and then it will display a windows showing you the list of all the errors. This report will also be saved as a text file in your model folder. If there is no error, you are lucky, ETABS will display Model has been checked. No warning Messages. You should always check your whole model, not just the selected objects. problems, just unlock the model, provide base supports and re run. An important tip here is; after closing the warning window, the BAD ELEMENTS will be selected by ETABS automatically. Do not click anywhere, just go to ASSIGN menu>group and assign these BAD ELEMENTS a group for selection. Now work on these errors by any mean you want. After you are done select the same group and check the model only for the selected group selection. In this way you will save time on model checking option. If you get warnings again (which will of course be less than previous warnings unless you made some blunder) assign them again the group and so on until you have zero warnings. Then check the model as whole for the final time. 3 SUPPORT YOUR BASE Always make sure, your model base has supports, pinned, fixed etc, otherwise your model is obviously unstable. When you run the analysis and find that the warning is related to negative stiffness values found in model, base support could be one of the ETABS does not check the supports for your model. If for example you have applied the supports for the base of model but have also drawn some lines in space with no supports, Checking the model will not return any errors, rather it will say there are no errors (unless you have other errors, like overlapping etc.) Page 2 of 28
Now, you have run the analysis and found that it displays warnings as shown here: These errors are about negative stiffness and loss of accuracy. Loss of accuracy error is okay, but if the no. of digits lost exceeds 11, ETABS stops the analysis process. Your model deformed shape will also be affected. It will only show the animation of the BAD ELEMENTS, because ETABS tries to give the deformed animation of all the model elements. Now if some element is ill conditioned and deforming like infinity then obviously other GOOD ELEMENTS deformation (small as compared with infinity) will be very negligible as compared to these BAD ELEMENTS deformation. So there is something wrong with your model. PROVIDE SUPPORTS. Page 3 of 28
II CONFIGURATION ERRORS 1 FRAME OUTPUT STATIONS Frame output stations is a useful command and it basically allows you to enter no. of segments or spacing of segments of a frame section (beam, column etc) at which you want the output (shear, moment, steel etc.). By default, there are 3 stations (start, mid and end) of a frame section. As you increase the no. of segments or decrease the spacing of segments, there will be obviously more and more calculations and your model run time will be increased. For a refined model, enter maximum spacing to be 150mm or 6. you must check this in ETABS help file. For example, only moments can be released on both ends, all other options (axial load, shear and torsion) can be only be released at only one end. It will now allow you to release on both the ends. Try changing shear force 2 and moment 3. Select the desired releases and press OK, or select No releases option to unselect all. You will see 2 green dots at either ends of frame for which you have released the shear, moments etc. This option is generally applied to a secondary beam resting on another (primary) beam. You can release both moments for this beam on both ends. 2 FRAME RELEASES Frame release is a special tool for releasing the frame for torsion, moment, shear, load etc. There is a limit on the combination and Page 4 of 28
3 ANALYSIS OPTIONS Before running your model, you should ensure that proper analysis options are selected to get the most out of your model. Two most important options are: i Dynamic Analysis ii P Delta Analysis If you are not analyzing your model for these options, unselect them. large error in model especially in member design (concrete, steel etc.). Generally the default values are okay, but you have to change the value of modulus of elasticity E for concrete which is equal to 57000 f c (psi) or 4700 f c (MPa). You may need to change the units to lb in or N mm for material properties as working in these units is easy in this dialogue box. Poisson s ratio 0.2 for concrete is ok. For normal weight concrete 150pcf density is generally used. You can change the data for your own needs. If you change the mass, weight will be automatically calculated and vice versa. Shear modulus is calculated automatically and coefficient of thermal expansion default value is okay. 5 FRAME SECTIONS & REINFORCEMENT DATA 4 MATERIAL PROPERTIES Properties of the materials used in the model should be checked carefully, because all the sections belong to a material and any error in material assignments can produce a very Make sure all your beam sections are defined as beams and all your column sections as columns. Verify the material property assigned to each section and check the section dimensions. You will not generally use the PROPERTY MODIFIERS at this stage. Based on the section dimensions, you can check the sectional geometric properties like moment of inertia etc. by clicking Section Properties button. For column sections, make sure that Reinforcement to be designed is selected for design purposes. Select the bar which is most likely to be used during construction. If you are unsure of the layout of reinforcement in your column, try symmetric pattern. For beams, concrete cover to rebar center is the distance from Page 5 of 28
concrete face to centroid of steel reinforcement. You should use at least two layers of steel reinforcement to calculate this. For example thickness (1.5 cover) (3/8 stirrup dia.) (0.75 #6bar) (0.5x1 for clear spacing b/w 2 layers of #6) = thickness 3.125. Axis 3 is the major axis and Axis 2 is the minor axis of the section. Page 6 of 28
6 FRAME PROPERTY MODIFIERS After checking the above mentioned checks, you can now check individual member checks. Most important of which is frame property modifiers. These values are used to modify the frame section properties such as stiffness, torsion, moment of inertia etc. Why we need to put these values? There are 2 answers to this question. First, the common case is that according to ACI 318 08 Section 9.5 Control of deflections, you need to compute stiffness of members for calculations of effective moment of inertia. These calculations are required for immediate deflections of members. To simplify the process ACI allows the use of such factors. Second reason is for slenderness analysis, which is generally not the case for many general type structures. Anyhow, following are some the frame property modifiers which are applicable for both mentioned reasons, safely. You should apply these factors after completing your entire model and if you delete a section and re draw, do not forget to assign again these modifiers. The most common and easy practice is to select all your beams, apply beam modifiers then select all the columns, apply column modifiers and so on for each type, before running your model. Page 7 of 28
Type J I3 I2 Beams 0.35 0.35 0.35 Columns 1.0 0.70 0.70 Walls Un cracked 8 Special Seismic Load Effects If you are following American codes i.e. UBC, IBC, ACI etc. you will need to define this option for the load combinations to be used in member designs, otherwise select do not include special seismic load effects. (Low rise) 0.70 Cracked (High rise/retaining) 0.35 Flat Slabs 0.25 Link Beams 0.75 Transfer Beams 0.5 If you put I3 = 0.35 for beam, ETABS will calculate Ieff = 0.35xIg used in deflection calculation according to chapter 9 of ACI. It will not calculate Icr or Mcr. It will just use this factor for Ieff. 7 PROPERTIES OF INDIVIDUAL FRAME SECTIONS You can check the details about a particular beam or column or any element by right clicking on it when model is unlocked. If you right click on a member when analysis had been run and the model is locked, it will show the details of analysis. To check the main properties in locked model, first select un deformed shape and then right click. Verify all your loading, support conditions, frame modifier factors, lengths etc. Unfortunately, you cannot change anything here, unlike REVIT STRUCTURES which is dynamic and parametric. Page 8 of 28
9 Mass Source For the calculation of seismic dead weight W which is generally equal to 100% dead load plus some portion of live load. All these loads are un factored. For example, according to UBC 97, this W is 100%D+25%L for warehouses. There are three options for mass source in ETABS. Generally second option is used which says From loads because you normally put all the loading (dead, live etc) in ETABS manually and the self weight is automatically included in one of your dead load case with multiplier 1, so this option is most suitable, but if you are doing advance analysis you can define the masses and select the first or third options accordingly. Other options should be default. For residential and light buildings live load can be used as low as 15%. Page 9 of 28
10 Static Load Cases Here define as many load cases for your model as you need e.g. dead loads, live loads, wind loads, seismic loads, thermal loads etc. To be simple define only one dead load with self weight multiplier 1(including finishes, dead, walls etc) and one live load. 11 SWAY FRAMES Before running the final analysis of concrete frame for designing in ETABS, always check the whole model (CTRL+A) or All option in select box, and click on view/revise overwrites in concrete frame design option and then check the box Element Type and select the sway type of your structure. Remember if you delete a section and redraw, always repeat this step. Page 10 of 28
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III TIPS/TRICKS 1 SAVING PROJECT Always save ETABS file in a separate folder because ETABS creates too many analysis files once you run the analysis of your model. Its unlike REVIT where each and every information is stored in only one file. Two most important files have extensions: 1 EDB (Main model file) 2 $ET (Text file, can be opened by notepad) If you want to send your model for review to anybody over internet or email or by any mean, only EDB file is sufficient. Once the reviewer runs the model on his computer same analysis files will be generated on his own machine. If you saved your model as ETABS version 9.7 and the reviewer has version 9.5, then the reviewer will not be able to open your project. In this case $ET extension file will be useful for him, so that he can import the text file data into ETABS, so its recommended to send both the files. See section Import Model. 2 IMPORT MODEL We will cover only one options here: 1 ETABS.e2k Text File 1 ETABS.e2k Text File This option is useful when you want to import newer version model into older version of ETABS, let s say version 9.7 model into version 9.5 ETABS. To do this, first open the $ET file in notepad. In fourth line you will see; PROGRAM ETABS VERSION 9.7.0, just change the version to your current ETABS version, save and close. Now import this file into ETABS. Important caution here; do not try to alter the contents of this file, it contains all the model information in text format. For example, you had 10m span of beam in original units of KN m and you change the units in this text file (7 th line) to KN mm, your beam span will be now 10mm instead of 10m in your model after importing. This trick can be useful to scale your model in some cases. Page 12 of 28
3 OPEN GL VIEW This option is very useful, if you have a huge model and want to view its extruded view, because it will be too slow to rotate a fully extruded model in ETABS modeling windows, instead apply the appropriate view settings and click on this option, it will allow you to rotate and move the model freely. You can even record the animation or capture the images and control the lighting etc. Page 13 of 28
4 DIMENSIONS You can draw the dimension lines from DRAW menu. After drawing the dimensions, did you ever worry about where your dimensions go after zooming in / zooming out your view? This is because the dimension lines option is not checked in view settings. Once you enable this option, all your hidden drawn dimensions will be available on screen. Just change the units and move mouse wheel up/down, your dimensions will be updated also. 5 MEASURE ANGLE or AREA Want to measure area or angle in ETABS like in AUTO CAD, yes you can do that. Page 14 of 28
Draw in this sequence: START, CENTER, END Now look at the status line. 6 FAST ZOOM Zooming in/out with mouse wheel is quite slow in ETABS. Goto dimension/tolerances settings and change the auto zoom step value to 100. Page 15 of 28
7 USER COMMENTS Received an ETABS model to review and interested which time of the day user worked on the model and where he stored it in his computer? If the user didn t clear the log before sending it you, you will be able to see all this data. Otherwise user can put his own comments here for you. Page 16 of 28
9 SUMMARY REPORT You can view the summary report of your model after analysis which includes important data like seismic calculations etc. in text format. 8 SET 3D VIEW Some times, on complex models, or where columns, struts or any other member is inclined, these inclined members will not shown on plan views. To work on them you must work on 3D view, but the original 3D view may not provide appropriate view/work plan for working on these type of members. For this reason, we use SET 3D view option. With these options, the view is still 3D but you can orient the view to front, top, back view etc. For example press xy button and enter zero in plan value and see what it shows. Page 17 of 28
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10 ENERGY DIAGRAM With the help of energy/virtual work diagram, we can easily check which member needs more cross section/strength/stiffness etc after running the analysis. Check ETABS help for more information. Page 19 of 28
11 DISPLAY DESIGN INFO After desiging for concrete structure, reinforcement values will appear on the members. ETABS gives you the options to view many reinforcement results, e.g. shear reinfrocement, longitudnal reinforcement etc. Always select your desired units to view the results e.g. in² or mm² for bar areas. For columns/beams you can check the reinforcement %age values also. Generally this option is used for columns. This value must not exceed by 3%, otherwise increase the column cross section. For general purpose (upto 3 storey construction) use %age value from 1% to 2% only. Always check for maximum no. of steel bars. For example, in general construction, you want to use #6 bars (0.44in² area) and you want that in no case, no. of bars be less than 8 which yields (8*0.44 = 3.52² area). So in this case your steel values from ETABS must not exceed 3.52, otherwise increase column size or take other measures (change your mind about dia of bars etc.) For shear reinforcement, ETABS gives value of Av/s, for example it gives you a value of shear reinforcement for column = 0.024. It means you can use #3bars 0.11*2/s = 0.0024 which yields spacing of approx. 9 inches. So provide #3 stirrups at 9 spacing for that specific column. bars. You may need to increase/decrease some reinforcement values to bring some columns into specific reinforcement groups. This all depends upon Engineer s experience and judgement. Sometimes, ETABS reinforcement may yield some weired values, which an experienced Engineer will adjust accordingly. You donot need to unlock the model for changing your section. You can define load combos, change sections and re run the design of concrete process only. After you have reached the final sizes and approximate reinforcement values by hit trial method, unlock the model and re run for the final time. You will notice some changes. Display design info after analysis and design This process is also very time consuming, you cannot have different reinforcement plan for each column and beam, but you will have to check the ETABS reinforcement values for each column and beam to see how many minimum no. of groups having same reinforcement can be generated. For example you may be able to get 3 types of groups for your columns having same size, same stirrups @ same spacing, and same no. of longitudnal Page 20 of 28
Change the design section If you right click on any member after design process, ETABS will show you different combinations used for that specific member. Each combination will have results at the locations you specified (FRAME STATIONS). By default, it will also highlight for you the combination with maximum area of steel (combination producing worst results). You can see the design summary, shear details, flexure details etc. Page 21 of 28
Longitudnal reinforcement values for columns 12 DISPLACEMENT & DRIFTS After analysis, you can right click on any node, joint to view its displacement values. For the lateral drifts click Lateral Drifts button. Page 22 of 28
The last two columns actually do not give the direct values of DRIFT X & DRIFT Y as shown, rather these are the values for DRIFT X/Storey Height and DRIFT Y/Storey Height, because we have to check according to UBC the limit for drift which is equal to 0.0020 or 0.0025xstorey height depending on time period (See UBC). Simplifying this, we get, 13 STOREY RESPONSE This is very useful and quick option and displays the results in graphical format, unlike standard tables form. Max drift allowed = UBC Value (0.0025 or 0.0020) x storey height Rearranging; Storey drift ratio = Actual drift / storey height So this storey drift ratio should not exceed 0.0025 or 0.0020 according to UBC. In ETABS it gives values of storey drift ratio not the actual drift, so you can easily see if ETABS value is exceeding 0.0025 or 0.0020 value of UBC or not?. Always remember to have appropriate units, before checking these results. Page 23 of 28
You can also check the detailed results in table format. Page 24 of 28
Now, a BUG here! After checking the results in table format as above, again select any node/joint and right click on it for displacement values as you did before. You will notice strange values for the same node. (worried!). This is a bug, whenever you use tables and again select the node it will give you strange values. Just refresh your model by zooming in/out with your mouse for a second or two and then right click on the node/joint. It will give you now the correct values. Showing WRONG VALUES with lot of NEGATIVE SIGNS in both windows. Page 25 of 28
14 LOCAL AXIS While defining structural frame sections and viewing their analysis results, it is important to know the axes of the members. Please also see ETABS help about this topic. For line sections: The 3 axis in ETABS is X axis. The 2 axis in ETABS is Y axis. The 1 axis is along the member length. For area sections, the z axis always depend on the path you chose to draw the area (clock wise or anti clock wise). Its recommended to keep this point in mind while working with area objects to draw all the areas in one orientations that is all clock wise or anticlock wise, because all areas will have same z direction either into the screen or coming out of the screen (depending on the orientation). It will also help in manual meshing (same axes for each small finite area). 15 MERGING POINTS If you are repeatedly receiving errors about points, lines and areas too close or overlapping, you can merge points within some tolerance (as high as 50mm). Page 26 of 28
16 VIEW OPTIONS There are no. of options available in ETABS to make your model look what you want. You can have beam section names, dimensions etc. You can fill the areas and extrude the view to have better looking 3D view. When extruding or using the fill object option, also use the section option instead of material option at left. This will show the colors of different sections used in the model, and will be easy to see where what section is changing. You can change the colors from frame sections, slab sections properties. Opening the same model file at another computer may give different colors. The most important option is at the left bottom corner, Apply to All Windows. Page 27 of 28
16 STATUS BAR Always keep an eye over the status bar to check which element is selected. 17 RESET MENU If you have any problem with menu, or you have disabled or have hidden some menu, reset the menu to solve the problem in Options menu>reset toolbars. Page 28 of 28