7. Seismic Design - University of Nebraska–Lincoln

1 Dynamic Loads Seismic Design Richard L Wood, 2018 Page 1 of 30 7. Seismic Design Lesson Objectives: 1) Describe code based Seismic Design in accordance with ASCE 7-16 and IBC 2012. 2) Compute mapped and Design spectral accelerations. 3) Categorize and identify the Seismic Design category and importance factors for building structures. 4) Qualitatively describe the equivalent lateral force method and identify when it is appropriate to use as well as its limitations. 5) Compute the Seismic coefficient and Design base shear values. 6) Quantitatively construct a site-specific response spectrum. 7) Compute and apply the vertical distribution of Seismic forces. Background Reading: 1) Read _____. Introduction: 1) Within Seismic Design , the Seismic load demand on a structure can be calculated via two simplistic methods.

2 A. _____ b. _____ 2) For structures which have regular configurations, uniform mass, and stiffness and strength distributions, the equivalent lateral force (ELF) method is most commonly used. a. The focus of second half of these notes is the ELF method. 3) However, sometimes linear or nonlinear dynamic time history analysis is performed to verify a particular Design . a. Typically done for _____ buildings, _____ structures, and when a _____ is required. Dynamic Loads Seismic Design Richard L Wood, 2018 Page 2 of 30 Modal Analysis with the Fundamental Mode Only: 1) The basis for the equivalent lateral force method common in most building codes (including ASCE 7-16) is on both modal analysis and response spectrum analysis concepts. 2) As previously expressed, the maximum base shear contributed for a particular mode n of a structure subjected to a horizontal base excitation can be written as: 3) Sketch: 4) In this above equation: 5) In the equation _____ is based on the effective _____.

3 Therefore one can define the effective _____ as: 6) Recall the relationship between the _____: 7) Substitution of these two relations into the first equation, one can write an expression for the maximum base shear for the nth mode as: Dynamic Loads Seismic Design Richard L Wood, 2018 Page 3 of 30 8) Therefore the maximum restoring force due to the nth mode, _____, developed within the structure can be written as: 9) Which can be rewritten as: 10) Using this equation above, the lateral _____ acting on each story (level i) can be expressed in terms of the story weight as: 11) Therefore it the response of a structure is dominated by its fundamental mode, one can estimate the maximum base shear and restoring forces induced by an earthquake as: 12) Let s revisit this when the Seismic values are ready to be distributed to each floor.

4 Dynamic Loads Seismic Design Richard L Wood, 2018 Page 4 of 30 Code Based Seismic Design Loads 1) Now let s focus on how to obtain Design loads as prescribed by ASCE 7-16 and IBC 2012. 2) Note a new revision within the live loads and Seismic hazard maps are incorporated into the revised version, ASCE 7-16. a. New to the ASCE 7-16 cycle is a new chapter on tsunami Design . Analysis Procedure Determination of Design Accelerations: 1) The first step in the determination of the Design acceleration is to determine the mapped maximum considered earthquake (MCE) spectral response values. a. MCE has a return period of _____. b. This is considered a rare event . c. This equates to a _____ probability of exceedance in _____ years. 2) Two mapped spectral values are determined, namely _____.

5 A. _____ considered to the short period acceleration at _____. b. _____ considered to the long period acceleration at _____. 3) These values can be obtained via IBC 2012 Figures (1) through (6), where a few examples are illustrated below in Figures 1-4. 4) An alternative approach to determine the mapped and Design accelerations is to use the online USGS tools. Screenshots and links are available in Figures 5 and 6. a. Note this is a very powerful tool and can also minimally compute a Design spectrum. 5) When determining the spectral accelerations, Seismic Design Category A may be permitted as an assignment if _____. a. This is considered as a building located in a region with a very low probability of experiencing damaging earthquake effects. Dynamic Loads Seismic Design Richard L Wood, 2018 Page 5 of 30 Figure 1.

6 Mapped maximum considered earthquake MCE spectral response acceleration at short periods for the US mainland (part one). Dynamic Loads Seismic Design Richard L Wood, 2018 Page 6 of 30 Figure 2. Mapped maximum considered earthquake MCE spectral response acceleration at short periods for the US mainland (part two). Dynamic Loads Seismic Design Richard L Wood, 2018 Page 7 of 30 Figure 3. Mapped maximum considered earthquake MCE spectral response acceleration at long periods for the US mainland (part one). Dynamic Loads Seismic Design Richard L Wood, 2018 Page 8 of 30 Figure 4. Mapped maximum considered earthquake MCE spectral response acceleration at long periods for the US mainland (part two). Dynamic Loads Seismic Design Richard L Wood, 2018 Page 9 of 30 Figure 5.

7 Online Seismic Design maps for US and its territories. Available at: Dynamic Loads Seismic Design Richard L Wood, 2018 Page 10 of 30 Figure 6. Example Seismic Design values for Italy. Other mapping tools are available for other countries. Available at: 6) After the mapped accelerations are determined, the _____ must be specified. The site class is based on the local soil conditions (commonly as _____) and can be classified as A-F in accordance with ASCE 7-16 Chapter 20. See Table 1. 7) A breakdown of the site classes include: a. _____ b. _____ c. _____ d. _____ e. _____ f. _____ Dynamic Loads Seismic Design Richard L Wood, 2018 Page 11 of 30 8) If the soil conditions are not known, a designer may use site _____ (as default) unless the building officials or geotechnical data determines site class E or F is present.

8 9) Using the mapped accelerations and site classification, the maximum considered earthquake spectral response accelerations can be adjusted for the site class effects at the short period (_____) and long period (_____). 10) Note the above values correspond to a hazard represented as _____ _____. 11) To determine the 5% damped of critical Design spectral response accelerations, denoted as _____ at short and long periods, respectively; the following equations from IBC can be utilized (using Tables 2 and 3): 12) The Design earthquake event considers an approximate return period of _____ and corresponds to a _____ probability of exceedance in _____ years. Dynamic Loads Seismic Design Richard L Wood, 2018 Page 12 of 30 Table 1.

9 Site classification table obtained from ASCE 7-16. Table 2. Short period site coefficient table obtained from IBC 2012. Table 3. Long period site coefficient table obtained from IBC 2012. Dynamic Loads Seismic Design Richard L Wood, 2018 Page 13 of 30 Analysis Procedure Seismic Design Category and Importance Factor: 1) The risk associated with each use of buildings and other structures is not constant. a. For example: one-story residential house versus a nuclear power facility. b. Various risk categories are defined in IBC Table and c. These can be found in ASCE 7-16 Table or similarly in FEMA P-749 Chapter 5 Table 3. d. Refer to Tables 4 and 5. 2) To account for the associated risks, Importance factors, _____, are introduced and can be found in ASCE 7-16 Table Refer to Table 6.

10 3) Seismic Design categories (_____) can be assigned and determined as a function of the mapped spectral accelerations and the risk category. 4) Definitions of Seismic Design categories is shown in Table 7. These definitions are obtained from FEMA P-749, but are very close to the brief descriptions in ASCE/IBC procedures. 5) The Seismic Design category will govern which analytical procedures are appropriate. Dynamic Loads Seismic Design Richard L Wood, 2018 Page 14 of 30 Table 4. Risk classification of representative buildings obtained from ASCE 7-16. Dynamic Loads Seismic Design Richard L Wood, 2018 Page 15 of 30 Table 5. Occupancy classifications for representative buildings obtained from FEMA P-749. Dynamic Loads Seismic Design Richard L Wood, 2018 Page 16 of 30 Table 6.