Transcript ADVANCED STEEL DESIGN

CIEG 301:
Structural Analysis
Loads, continued
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
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 “oneway” 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
10’
14’
30’
20’
Live Loads
 “Moving loads”
 Loads vary in magnitude and location
 Examples
 In buildings:
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People
Furnishings
Materials
Cranes
Automobiles
 In bridges:
 Vehicular traffic
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
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:
15
 L
 0.25 
RF
K LL A T
 > 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
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)
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
Amer. Society of Civil Engrs.
(ASCE) 7-02 Wind Map
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
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….
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)