Sample Problem ASCE 7-05 Seismic Provisions A Beginner’s Guide to ASCE 7-05 Dr. T. Bart Quimby, P.E. Quimby & Associates www.bgstructuralengineering.com Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

The Problem Definition The wood framed office building shown here is to be constructed in a “suburban” area in Juneau, Alaska out near the airport. The site conditions consist of deep alluvial deposits with a high water table. Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05 Other Given Data Roof DL = 15 psf Typical Floor DL = 12 psf Partition Load = 15 psf Snow Load = 30 psf Exterior Wall DL = 10 psf Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

Determine the Seismic Design Category The building is in Occupancy Category II Get SS and S1 from the maps or online Using USGS software with a 99801 zip code: SS = 61.2%; S1 = 28.9% The building Site Class is D From Tables Fa = 1.311; Fv = 1.822 Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

Seismic Design Category continued…. Determine SMS and SM1 SMS = FaSS = 1.311(0.612) = 0.802 SM1 = FvS1 = 1.822(0.289) = .526 Determine SDS and SD1 SDS = (2/3) SMS = 2(0.802)/3 = 0.535 SD1 = (2/3) SM1 = 2(0.526)/3 = 0.351 Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

Seismic Design Category continued…. SD1 = 0.351 SDS = 0.535 Use Seismic Design Category D Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

Categorize the Plan Irregularities The building has re-entrant corners (type 2) since the projection is more than 15% of dimension 0.15(40’) = 6’ < 10’ and 0.15(60’) = 9’ < 30’ No Vertical Irregularities Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

Determine the Analysis Method Use ELF Method Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05 Determine R, I, and Ta From Table 5.2.2, R = 6.5 for bearing wall systems consisting of light framed walls with shear panels. Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05 Determine I and Ta From Table 11.5-1, I = 1.0 Determine the approximate fundamental period for the building (Section 12.8.2.1) Ta = 0.020(40’)3/4 = .318 sec. Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05 Determine Cs From section 12.8.1.1: Cs = SDS/(R/I) = .535/(6.5/1) = 0.0823 lower limit = 0.01 TL = 12 (Figure 22-17) Upper limit = SD1/(T(R/I)) = .351/(.318*6.5/1) Upper limit = 0.169 USE CS = 0.0823 Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

Determine Building Weight Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

Compute the Base Shear, V V = CsW = 0.0823(299.74 k) = 24.67 k This is the total lateral force on the structure. Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

Compute the Vertical Distribution Base Shear, V = 24.67 kips k = 1 Level wx hx wxhxk Cvx Fx   (k) (ft) (ft-k) Roof 67.3 40 2692 0.367 9.05 4th floor 77.48 30 2324.4 0.317 7.81 3rd floor 20 1549.6 0.211 5.21 2nd floor 10 774.8 0.106 2.60 Sum: 299.74 7340.8 1.000 Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

Typical Level Horizontal Distribution Load is distributed according to mass distribution. Since the loading is symmetrical, each of the two supporting shear walls receives half the story shear. Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05 Determine the Design Shear Force for the Shearwall on Grid A and the 2nd Floor Story shear from structural analysis is 11.03 kips Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05 Compute E There is no Dead Load story shear so E = DQE = 1.0 (11.03 k ) = 11.03 k D = 1.0 since the stories resisting more than 35% of the base shear conform to the requirements of Table 12.3-3 (other). QE = 11.03 k Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

ASCE 7 Load Combinations See ASCE 7-05 2.3 & 2.4 LRFD 5: 1.2(0) + 1.0(11.03) + (0) + 0.2(0) = 11.03 k 7: 0.9(0) + 1.0(11.03) = 11.03 k ASD 5: (0) + 0.7(11.03) = 7.72 k 6: (0) + 0.75(0.7(11.03)) + 0.75(0) + 0.75(0) = 5.79 k 8: 0.6(0) + 0.7(11.03) = 7.72 k Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05

ASCE 7-05 Load Combinations Combinations 3 & 4 have E in them. For the wall shear: D = L = 0 E = 11.23 k Design Wall Shear = 11.23 k Seismic Provisions Example – A Beginner’s Guide to ASCE 7-05