Changes to the 2016 Edition of ASCE 7. J. G. (Greg) Soules, P.E., S.E. Principal Engineer CB&I Inc. Vice Chair, ASCE 7-16 Main Committee

Transcription

1 Changes to the 2016 Edition of ASCE 7 J. G. (Greg) Soules, P.E., S.E. Principal Engineer CB&I Inc. Vice Chair, ASCE 7-16 Main Committee

2 While using ASCE 7, please keep in mind the old adage A camel is a horse designed by committee.

3 Changes to Chapter 1 (General) We have changed the title of the document! ASCE 7-16 will be known as: Minimum Design Loads and Associated Criteria for Buildings and Other Structures

4 Changes to Chapter 1 (General) ASCE 7 has for quite some time contained more than just design loads. Chapter 1 contains a Risk Category Table, Strength and Stiffness requirements, performance-based design requirements, serviceability requirements, and General Structural Integrity provisions. The flood chapter makes an explicit declaration that the effects of scour and erosion be considered. The rain chapter places requirements on the drainage and consideration of ponding instability.

5 Changes to Chapter 1 (General) ASCE 7 has for quite some time contained more than just design loads. The Atmospheric Ice section contains a specific design procedure for the consideration of ice loads. The Earthquake chapters contains requirements on analysis, structural configuration directives, and prohibits some structural systems and detailing methods. Appendix C contains serviceability requirements. The scope and commentary are also revised to reflect this change in title.

6 Changes to Chapter 1 (General) Reliability Basis for use in Performance-Based Design moved from Commentary to Provisions.

7 Changes to Chapter 1 (General) The current load combinations and companion resistances used in our prescriptive design procedures are intended to provide reliabilities similar to those shown in the target reliability table. The ASCE 7 committee is looking at these reliabilities in the context of many of our existing environmental loads. Later in the presentation, you will see how the reliability impacts wind loads for Risk Category IV structures. Performance-based procedures must also demonstrate that they provide a reliability not less than these target reliabilities.

8 Changes to Chapter 2 (Load Combinations) A new non-mandatory appendix (Appendix E) has been added on: Performance-Based Design Procedures for Fire Effects on Structures

9 Changes to Chapter 2 (Load Combinations) Appendix E is not a mandatory part of the standard. Provides procedures for performance-based design and evaluation of structures for fire conditions that result in fire-induced effects on a structure s members and connections. Constitutes an alternative methodology to meet project design requirements, as is permitted by Section and the alternate means and methods of construction provision in the building codes. Does not provide for design using standard fire resistance design with prescriptive methods, nor does it address explosions.

10 Changes to Chapter 4 (Live Load) Table 4-1 (Minimum Uniformly Distributed Live Loads)had no fewer than 16 very detailed footnotes in ASCE In ASCE 7-16, Table 4-1 has NO footnotes. Footnotes were not deleted. They simply were moved over to the provisions. This change makes use of Table 4-1 easier and increases the clarity of the requirements.

11 Changes to Chapter 4 (Live Load) Old Table 4-1

12 Changes to Chapter 4 (Live Load) New Table 4-1

13 New Chapter 6 (Tsunami) Significant loss of life was experienced during the 2004 Indian Ocean Tsunami and the 2011 Tohoku (Japan) Tsunami. Significant populations are at risk from Tsunamis in California, Oregon, Washington, Hawaii, and Alaska. ASCE 7-10 is silent on how to deal with Tsunami risk.

14 New Chapter 6 (Tsunami) The Tsunami Loads and Effects Subcommittee of the ASCE/SEI 7 Standards Committee has developed a new Chapter 6 - Tsunami Loads and Effects for the ASCE 7-16 Standard, which has been approved. ASCE 7-16 to be published in 2016 Tsunami Provisions would then be referenced in IBC 2018 State Building Codes of AK, WA, OR, CA, and HI ~ 2020 ASCE will be publishing two design guides in 2016 with design examples.

15 New Chapter 6 (Tsunami) The following buildings and other structures located within the Tsunami Design Zone shall be designed for the effects of Maximum Considered Tsunami including hydrostatic and hydrodynamic forces, waterborne debris accumulation and impact loads, subsidence, and scour effects in accordance with this Chapter: a. Tsunami Risk Category IV buildings and structures; b. Tsunami Risk Category III buildings and structures with inundation depth at any point greater than 3 feet, and c. Where required by a state or locally adopted building code statute to include design for tsunami effects, Tsunami Risk Category II buildings with mean height above grade plane greater than the height designated in the statute, and having inundation depth at any point greater than 3 feet. Exception: Tsunami Risk Category II single-story buildings of any height without mezzanines or any occupiable roof level, and not having any critical equipment or systems need not be designed for the tsunami loads and effects specified in this Chapter.

16 New Chapter 6 (Tsunami) For the purposes of this chapter, Tsunami Risk Categories for buildings and other structures shall be the Risk Categories given in Section 1.5 with the following modifications: 1. State, local, or tribal governments shall be permitted to include Critical Facilities in Tsunami Risk Category III, such as power-generating stations, water treatment facilities for potable water, waste water treatment facilities and other public utility facilities not included in Risk Category IV. 2. The following structures need not be included in Tsunami Risk Category IV and state, local, or tribal governments shall be permitted to designate them as Tsunami Risk Category II or III: a. Fire stations and ambulance facilities, emergency vehicle garages b. Earthquake or hurricane shelters c. Emergency aircraft hangars d. Police stations that do not have holding cells and that are not uniquely required for postdisaster emergency response as a Critical Facility. 3. Tsunami Vertical Evacuation Refuge Structures shall be included in Tsunami Risk Category IV.

17 Changes to Chapter 7 (Snow) Updated Map SEA state snow maps for Oregon, Colorado, Idaho, Montana, New Mexico, and Washington incorporated. Tables created with values for major cities within state with reference to the original SEA document. Eliminates conflicts between previous ASCE 7 map and SEA maps. Eliminates case study (CS) areas in these states.

18 Changes to Chapter 7 (Snow)

19 Changes to Seismic Too many changes to the seismic provisions to go through all of them. We will look at the changes to the horizontal ground motions and the new site factors. We will briefly look at the changes to Chapter 16.

20 New Seismic Ground Motions Based on USGS s 2014 National Seismic Hazard Maps. New/updated fault characterizations. New Ground Motion Prediction Equations. Changes resulting from updated soil amplification factors. The Problem at softer soil sites whose seismic hazard is dominated by large magnitude events. The Light at the End of the Tunnel Project 17.

21 New Seismic Ground Motions The ground motions are changing! AGAIN!

22 New Site Factors F a & F v

23 New Site Factors F a & F v Site factors adjusted to a reference site condition of v s = 760 m/s (instead of Site Class B) and to reflect more recent knowledge and data pertaining to site response. Some values have gone up and some have gone down. As before, if site class is unknown, use Site Class D EXCEPT minimum value of F a is 1.2. Section will be discussed in a moment.

24 DERIVED SITE FACTOR STUDY PROPOSED APPROACH TO MINIMIZE POTENTIALLY UN-CONSERVATIVE SEISMIC LOADS OF THE ELF AND MRSA DESIGN METHODS Provisions Update Committee Meeting - September 15-16, 2014 Charlie Kircher Kircher & Associates, Palo Alto, CA Nicolas Luco USGS, Golden CO

25 Background Root Cause of the Problem Section 11.4 of ASCE 7-10 (ASCE 7-16) - Use of only two response periods (0.2s and 1.0s) to define ELF (and MRSA) design forces is not sufficient, in general, to accurately represent response spectral acceleration for all design periods Reasonably Accurate (or Conservative) When peak MCE R response spectral acceleration occurs at or near 0.2s and peak MCE R response spectral velocity occurs at or near 1.0s for the site of interest. Potentially Non-conservative When peak MCE R response spectral velocity occurs at periods greater than 1.0s for the site of interest (e.g., softer soil sites whose seismic hazard is dominated by large magnitude events).

26 Example ELF Design Spectrum based on ASCE 7-16 (Revised) Site Factors M8.0 earthquake ground motions at R X = 8.5 km, Site Class DE ELF Design Spectrum C s x (R/I e ) = min[s DS, S D1 /T] MCEr - BC (Vs,30 = 2,500 fps) MCEr - DE (Vs,30 = 600 fps) Design DE (Vs,30 = 600 fps) ELF DE (Vs,30 = 600 fps) Spectral Acceleration (g) Conservative Ground Motion Values S DS = 2/3F a S s = 2/3 x 0.9 x 1.56g = 0.94g S D1 = 2/3F v S 1 = 2/3 x 1.85 x 0.70g = 0.86g Period (seconds) ASCE 7-16 (Revised) F a = 0.9 = ( )/2 F v = 1.85 = ( )/2 T s = 0.85s Non-Conservative

27 How to fix the Problem Multipoint spectrum is the ultimate solution but is not ready for the 2016 Edition (Future Project 17). Factors could be applied to the current ground motions to make the correction (fix rejected by ASCE 7). Require site-specific response analysis in the following situations (fix accepted by ASCE 7 in Section ): structures on Site Class E sites with S S greater than or equal to 1.0. structures on Site Class D and E sites with S 1 greater than or equal to 0.2. Site-specific studies will become more common place in large portions of the country.

28 Background - Map of T L Regions (and Relationship to Earthquake Magnitude) (Chapter 22, ASCE 7-05, ASCE 7-10 and ASCE 7-16)

29 Project 17 Joint BSSC USGS project to develop consensus between earth science and geotechnical communities as to basis for maps in ASCE 7-22 Initiated in February 2013 Will complete mid-year 2018 with publication of preliminary maps for ASCE 7-22

30 Multi-period spectra Project 17 Precision vs. Uncertainty Small changes in earth science mean radical change in contours Maps change significantly every cycle Changes are not statistically significant Acceptable Collapse Risk Use and definition of deterministic caps

31 Chapter 16 Changes Big Picture ASCE 7-10 Chapter 16 covered linear and nonlinear response history analysis Conducted at Design Earthquake shaking Acceptance values = 2/3 of failure level demands Alternate procedure to ELF, RSA ASCE 7-16 Chapter 16 limited to nonlinear response history analysis Linear moved to Chapter 12 Conducted at MCE R shaking Acceptance values taken at statistical probability of failure Supplementary procedure to ELF, RSA, or linear response history

32 Motivation Performance-based Seismic Design of Tall Buildings

33 Important New Concepts Acceptance based on local and global behavior Failure probability tied to overall performance goal (10% probability collapse given MCE R )

34 Changes to Wind New/Revised Wind Maps New MRI/Map for Risk Category IV Structures based on reliabilities consistent with new Table a. Revised all maps to incorporate more stations, additional years of data, and updated analysis/modeling methods.

35 Changes to Wind

36 Changes to Wind

37 Changes to Wind

38 Changes to Wind

39 Questions?