Loads Copyright Prof Schierle 2012 1Loads Prof Schierle 1Portions of this document reproduce sections from the 2003 International Building Code, International Code Council, Falls Church, Virginia. Reproduced with permission. All rights reserved.Also included are USGS figures, courtesy US Geological Survey Read Structure and Design chapter 2Loads Copyright Prof Schierle 2012 2Loads Prof Schierle 2reading room = 60 psfstack room = 150 psfLibrarylight = 125 psf heavy = 250 psf Manufacturingfixed seating = 60 psf movable seating = 100 psf Assembly50 psfOffice40 psf Residential and schoolsASCE 7 Table 4.1 excerpts of common live loadsASCE 7, page 10Live load reductionSince it is unlikely large members are fully loaded, ASCE 7 allows these live load reductions (except for public spaces and LL ≥ 100 psf):For members supporting ≥ 600 sq. ft.Reduction shall not exceed: 50% for members supporting 1 floor, 60 % for members supporting 2 or more floorsLoads Copyright Prof Schierle 2012 3Loads Prof Schierle 3LoadStructures must resist various loads, such as gravity and wind.Loads may change rapidly or slowly. For example:• Furniture are moved gradually• Wind load changes rapidlyLateral load (act horizontally):• Wind• Earthquake• Soil pressure on retaining wallDesign load is based on probability (50-year wind for example)Load types:1 Dead load (DL -structure and permanently attached items)2 Live load (LL - other load, people, furniture, snow, etc)3 Distributed load (snow drift, etc.)4 Uniform load (uniformly distribution)5 Point load (concentrated load)6 Partial beam load causing greater bending than full load7 Negative support bending reduces positive bending8 Static load (at rest)9 Impact load (moving object hitting a structure)10 Dynamic load (cyclic load: earthquakes, wind gusts, etc.)Classification as DL and LL is because:• Seismic load is effected by dead load• For continuous beams, etc. partial load may be more criticalthan full load. Thus LL is assumed only on partial beam.Loads Copyright Prof Schierle 2012 4Loads Prof Schierle 4Snow live loadLoads Copyright Prof Schierle 2012 5Loads Prof Schierle 5Static loadDynamic loadLoads Copyright Prof Schierle 2012 6Loads Prof Schierle 6Impact loadDon’t try it !Loads Copyright Prof Schierle 2012 7Loads Prof Schierle 7World Trade Center TragedyCollapse scenarioImpact load of the aircrafts and the ensuing inferno weakened the effectedfloor structure, causing floors above to collapse into itImpact load of the falling floors caused failure of the structure below(impact effect amplifies load greatly)The ensuing domino effect brought theentire structure downLoads Copyright Prof Schierle 2012 8Loads Prof Schierle 8World Trade Center New YorkArchitect: Minoru Yamasaki Engineer: Skilling / RobertsonThe World Trade center had a Framed Tube structure, composed ofclosely spaced columns (~ 1 meter ~ 3 ft) combined with moment resisting beamsat each floor to provide a lattice wall toresist gravity and lateral loads.1. Axonometric view of one tower2. Floor framing3. Prefab steel element of framed tube4. Typical columnLoads Copyright Prof Schierle 2012 9Loads Prof Schierle 9Load Path andTributary Load• Load path is the path load travels from where it acts to where it is resisted • Tributary load is the load acting on a member(needed to design it)It is convenient to visualize andcompute load on a strip of unit width (1 foot or 1 meter)For example:• 1’ slab, resting on • 1’ wall, resting on• 1’ footing, resting on•1’ soilLoads Copyright Prof Schierle 2012 10Loads Prof Schierle 10Lateral wind loadLoad path: A > B > C• A wind wall • B floor and roof diaphragms • C shear wallsTributary load:A Wind wall resists wind pressureB Floor/roof diaphragms resist wind wall load(½ of wall above & ½ of wall below) C Shear walls resist ½ each (2 walls) offloor and roof diaphragmsLoads Copyright Prof Schierle 2012 11Loads Prof Schierle 11Load Path1 Slab / wallSlab rests on walls2 Deck / joist / wallDeck rests on joistsJoists rest on walls3 Slab / beam / wallSlab rests on beamsBeams rest on walls4 Deck / joist / beam / wallDeck rests on joistsJoists rest on beamsBeams rest on walls5 Deck / joist / beam / girder / postDeck rests on joistsJoists rest on beamsBeams rest on girdersGirders rest on post (column)All supported by footingLoads Copyright Prof Schierle 2012 12Loads Prof Schierle 12Tributary load: deck / joist / beam / columnAssume Uniform load w = 80 psfJoist spacing e = 2’Joist span L1 = 12’Beam spans L2 = 10’L3 = 20’Find load path and tributary loadLoad path: plywood deck > joist > beam > columnsTributary loads:Uniform joist loadwj= w e = 80 psf x 2’ wj = 160 plfBeam load (assume uniform load due to narrow joist spacing)wb= 80 psf L1/2 = 80 psf x 12’ /2 wb= 480 plfPost reactionsRa= wb L2 / 2 = 480 plf x 10 /2 Ra= 2,400 #Rb= wb(L2+L3)/2 = 480 (10+20) / 2 Rb= 7,200 #Rc= wbL3 / 2 = 480 x 20 / 2 Rc= 4,800 #Loads Copyright Prof Schierle 2012 13Loads Prof Schierle 13Tributary loadThree-story building1 Isometric view2 Exploded visualization3 DimensionsLoad path:Wind wall > diaphragms > shear wallsAssume Wind pressure P = 20 psfShear wall shear (2 walls)Third floorV3 = 20 psf x 100’ x 5’/1000 V3 = 10 kSecond floorV2 = 20 psf x 100’ x 15’/1000 V2 = 30 kFirst floor = base shear VV = 20 psf x 100’ x 25’/1000 V = 50 kNote:Each diaphragm resists wind pressure from half thewall above and below. Lower half of 1stfloor resistedby footing; hence shear walls don’t resist lower half.Loads Copyright Prof Schierle 2012 14Loads Prof Schierle 14BeamP = 16 k Tributary load3 Concrete slab / beam / wallSlab span L = 10’, t = 5” Beam span L = 30’LL = 20 psfDL = 70 psf (including beam DL) = 90 psfBeam load w = 90 psf x10’ / 1000 w = 0.9 klfWall reaction R = 0.9 klf x 30’ / 2 R = 13.5 k4 Concrete slab on metal deck / joist/ beamDeck