Class 3 Allowable Stress and Strength Design Methods Load Combinations

1. Class 3Allowable Stress andStrength Design MethodsLoad Combinations

2. Symbols and Notation Building Design – Fall 2007

3. Strength Design • Use ASCE 7 – Chapter 2 and commentary as reference • Also referred to as LRFD by steel and wood industries • Applies to limit states • Most unfavorable combination is to apply • Can not be used in combination with ASD Building Design – Fall 2007

4. Strength Design Combinations Building Design – Fall 2007

5. Strength Design Combinations • Flood load exceptions Building Design – Fall 2007

6. Allowable Stress Design • Based on allowable stresses developed by material industries • Most unfavorable combination is to apply • Caution about allowances for 1/3 increase in stress with load combinations • There may be a .75 load combination reduction factor Building Design – Fall 2007

7. ASD Combinations Building Design – Fall 2007

8. ASD Combinations • Flood load exceptions Building Design – Fall 2007

9. Extraordinary Events • Very low probability events – 10,000 MRI event or lower • Examples: • Fire • Explosion from liquid or gas • Bombs • Subsidence • Tornadoes Building Design – Fall 2007

10. Extraordinary Events • Design concept is to ensure performance of key load bearing elements is sufficient to withstand the event • Prevent progressive collapse • Structures designed with sufficient continuity and ductility such that alternative load paths can develop Building Design – Fall 2007

11. Extraordinary Events • Ref. ASCE 7 C1.4 – design strategies to prevent progressive collapse: • Alternate Path Method • Specific Local Resistance Method • Examples of design strategies that improve resistance to extraordinary events Building Design – Fall 2007

12. Design strategies for extraordinary events • Plan layout of walls and columns – reduce spans • Put returns on walls to increase stability • Reinforce floor slabs so they could span in more than one direction • Load-bearing interior partitions • Catenary action of floor slab • Beam action of walls • Redundant structural systems • Ductile detailing • Reinforce to resist blast and load reversal • Special moment-resisting frames Building Design – Fall 2007

13. Extraordinary load combinations • For checking residual capacity following an event • (0.9 or 1.2)D + (0.5L or 0.2S) + 0.2W • For checking capacity to withstand an event • 1.2D + Ak + (0.5L or 0.2S) • (0.9 or 1.2)D + Ak + 0.2W • Ak = value of the extraordinary load Building Design – Fall 2007

14. Examples • Use the CCM example case study A bldg in coastal A zone • Use the same bldg but change loads to vertical and add snow for LI, NY • Use the same bldg but put a large hole in bldg from flood borne debris – object weighs 4000# and is size of car Building Design – Fall 2007

15. Event Probabilities Building Design – Fall 2007

16. Determining probability • Pn = 1 – (1 – Pa)n • Pa = annual probability • n = length of period of interest • Return periods for design events are given in the appropriate ASCE section Building Design – Fall 2007

17. ASCE 7 Design level MRI • Wind – 50-yr MRI • Flood – 100-yr • Seismic - ~475-yr • Snow – 50-yr • Ice – 50-yr • Rain – varies by jurisdiction Building Design – Fall 2007

18. Performance based design • Determine the performance required of a building during and post event • Determine the intensity of the design event being designed for • Base design on the building performance during the ‘design event’ – may exceed bldg code requirements by significant margin Building Design – Fall 2007

19. Homework • Read Chap 2 ASCE 7 • HW 1 on class web site – due 9/19/07, email it to me or turn in during class next week Building Design – Fall 2007