CIEG 301: Structural Analysis

1. CIEG 301:Structural Analysis Loads, continued

2. Load Transfer and Load Distribution • Considered a typical building framing plan • Work from top down • Determine tributary widths and tributary areas as appropriate • Applicable for dead load and live load • Similar approach used for all load types • Work from point of load application to point where load is transferred out of the structure

3. 10’ 14’ 30’ 20’ Load Transfer and Load Distribution (cont’d) • When members supporting the slab are oriented in one direction: • One-way slab • When members supporting the slab are oriented in both directions: • Use L2/L1 (long dimension / short dimension) to determine if “one-way” slab or “two-way” slab • L2/L1> 2  one-way slab, otherwise two-way slab • If two-way slab, draw lines at 45 degrees to determine tributary area

4. Live Loads • “Moving loads” • Loads vary in magnitude and location • Examples • In buildings: • People • Furnishings • Materials • Cranes • Automobiles • In bridges: • Vehicular traffic

5. Live Loads, Continued • In buildings, typically applied to the structure as a uniformly distributed load • Load is applied to all or part of the structure to maximize load effects • Typical magnitudes of live load: • Table 1-4

6. Live Load Reduction • For large floor areas, there is less probability that the entire floor will be loaded • Live load reduction factors are used to reduce the applied load • In English units this factor is: • > 0.5 for members supporting one floor • > 0.4 for members supporting more than one floor • KLL = member type coefficient (for interior columns, KLL = 4) • AT = tributary area • L = LoLRF • Lo = original live load, > 100 lb/ft or reduction NA • Reduction NA for public assembly spaces, garages, and roofs

7. Snow Load • Historical data is used to determine maximum snow depths over 50-year recurrence interval for a specific location • This gives the ground snow load, pg • pg is modified to give the roof snow load • For “flat” roofs (< 5% slope), • p = 0.7CeCtIpg • Ce = exposure factor (0.8 for unobstructed; =1.3 for sheltered urban) • Ct = thermal factor (1.0 for normal heat; 1.2 for unheated) • I = importance (0.8 for agriculture / storage; 1.2 for hospitals)

8. Wind Load • Kinetic energy generated by wind: • KE = 0.5rV2 • V = velocity (wind speed) • r = air density • Kinetic energy becomes potential energy (pressure) when a structure blocks the air flow

9. Amer. Society of Civil Engrs. (ASCE) 7-02 Wind Map

10. Wind Pressure (qz) • 0.5rV2 is converted into wind pressure (qz) • qz = 0.00256KzKztKdV2I [lb/ft2] • Kz = exposure coefficient (depends on structure height and ground terrain) • Kzt = exposure topography coefficient • Kd = direction coefficient (equal to 1.0 when wind is the only load considered) • V2 = velocity in mph • I = importance of the structure

11. Wind: Design Loads • The design pressure for wind loading is the difference between the external and internal pressure • p = qGCp – qh(GCpi) • q = qz for the windward wall at height z • qh = qz for the leeward wall, side wall, and roof, where z = h = the mean height of the roof • G = gust effect factor = 0.85 for rigid structures • Cp = pressure coefficient • GCpi = internal pressure coefficient (+ 0.18 for fully enclosed buildings, + 0.55 for partially enclosed, 0 for open) • Handout: Selected sections of Chapter 6 (from ASCE 7-02) • Example….

12. Homework • Due Sept. 7th / Next Thurs. • 3 Problems: • Determine wind loads acting on roof and leeward wall of the in-class example • From chapter 1 of textbook: • 1-2 (dead load) • 1-10 (live load)