Presentation on theme: "Impact Load Live loads specified in codes do account for ordinary impact loads When structural members are subject to unusual vibration or impact we."— Presentation transcript:
1 Impact LoadLive loads specified in codes do account for ordinary impact loadsWhen structural members are subject to unusual vibration or impact we have to account for them outside the code specs
2 Minimum % increase in live load on structural members due to impact Type of memberSource of ImpactPercent increaseSupportingElevators and elevator machinery100Light machines, shaft, or motor driven20Reciprocating machines or power-driven units50HangersFloors or balconies33
3 Crane Runway Loads Structures supporting cranes: Maximum wheel loadsAllowance for impactMultiple cranesTraction and braking forcesUse of crane stopsCyclic loading / FatigueCrane live load is its fully rated capactity
6 Crane load Max vertical wheel load Monorail, cab operated, remote operatedincreased by 25% for impactPendant operated overheadIncreased by 10% for impactImpact increases do not have to be applied to supporting columns, only runway
7 Crane Lateral Loads Electic powered trolleys ≥ 20% (crane rated load + trolley weight + hoist weight)Assume applied by wheels at top of railsActs normal to the railsDistributed, as appropriate to stiffness of rail supportBridge or monorail with hand-gearingNo need for lateral load increase
8 Crane Stop Forces Runway must be designed for stop forces Velocity of crane at impact taken into accountFatigue and serviceability concernsAISC Design Guide 7AISE Standard No. 13
9 Restraint LoadsCaused by changes in dimensions/geometry of structures due toBehavior of materialType of framingDetails of constructione.g.Foundation settlementTemperature changesShrinkage restrained by adjoining structures
10 Combined Loads Loads may act simultaneously Building codes specify various combinations that must be consideredDepends on whether allowable stress design (ASD) or Load and Resistance Factor Design (LRFD) is usedSEI/ASCE 7-02 provides guidance.
11 Load Sources D = dead load L = live floor load, including impact Lr = roof live loadS = roof snow loadR = rain load (initial rainwater or ice, exclusive of ponding)W = wind loadE = earthquake loadT = restraint load
12 ASD Loads (SEI/ASCE 7-02) D D + L + T D + (Lr or S or R) 0.75 [ L + (Lr or S or R) + T ] + D0.75 (W or 0.7E) + D0.75 [ L + (W or 0.7E) + (Lr or S or R) ] + D0.6D + W0.6D + 0.7EBecause E was calculated for LRFD it is reduced by 0.7 for ASD design.
13 LFRD Design Loads (SEI/ASCE 7-02) 1.2(D+T) + 1.6L + 0.5(Lr or S or R)1.2D + 1.6(Lr or S or R) + (L or 0.8W)1.2D + 1.6W + L + 0.5(Lr or S or R)1.2D + E + (L or 0.2S)0.9D + 1.6W0.9D + E
14 Fire Protection International Building Code International Code Council, Falls Church, VANFPA 5000, Building Construction and Safety CodeNational Fire Protection Association, Quincy, MANational Building Code of CanadaNational Research Council of Canada, Ottawa, ONOr local code
15 Combustible/Non-combustible Most fires are accidental or carelessnessStart small and require fuel and ventilation to growNoncombustibles (concrete, steel, brick) are not fuelCombustibles (paper, wood, plastics) are fuel
16 Fire Loading and Fire Severity Fire loading is the amount of fuel, measured in equivalent pounds of wood per square foot of floor areaFire severity is the duration of the fire, in hours of equivalent fire exposureMore modern approaches of fire load are expressed in terms of potential heat energyFire loading correlates well with fire severity
17 Fire LoadingReasonable estimate for conventional wood frame construction:7.5 – 10 lb/ft2Reasonable heavy timber estimate12.5 – 17.5 lb/ft2Consequently building codes limit permitted size (height and area) of combustible buildings more than non-combustible buildings.BUT ventilation is an important factor as well.
19 Occupancy Fire Loads and Severity Type of OccupancyFire Load (lb/ft2)Fire Severity (hrs)Assembly5-100.5 – 1Business0.5-1EducationalHazardousVariableIndustrialLow hazard0-100-1Moderate hazard10-251-2.5InstitutionalMercantile10-201-2ResidentialStorage10-301-3
20 Fire ResistanceFire Resistance: Relative ability of construction assemblies to prevent spread of fire to adjacent spaces, and to avoid structural collapseFire resistance requirements are a function of occupancy and size (height and area)Fire resistance is determined experimentallyASTM E 119Uses “standard” fire exposureSpecified in terms of time of exposure
21 Fire Resistance Time during which an assembly continues to prevent spread of fire,does not exceed certain temperature limits, andSustains its structural loads without failureTypically expressed in hoursFire Resistance Directory, Underwriters LabFire Resistant Ratings, American Insurance Services Gp.Fire Resistant Design Manual, Gypsum Association
23 FireproofNo building is fireproof.Avoid this term
24 Effect of Temperature on Steel In general, steel can hold 60% of yield strength at 1,000 FFailures rarely occur because during a fire building is rarely loaded at design load.This is not recognized in the code – structures are assumed to be fully loaded during testing.Thus, when building codes specify fire resistant construction, fire protection materials are required to insulate the structural steel.
26 Fire Protection Materials GypsumAs a plaster, applied over metal lathe or gypsum latheAs wallboard, installed over cold-formed steel framing or furringEffectiveness can be increased significantly with lightweight mineral aggregates (vermiculite, pearlite)Mix must be properly proportioned and applied in required thickness and the lathe correctly installed
27 Gypsum 3 kinds: Type X: Proprietary Regular, Type X, and proprietaryType X:Specially formulated cores for fire resistance.ProprietarySuch as Typc C, even greater fire resistanceType of wall board must be specified clearly.Type and spacing of fasteners (and furring channels if applicable) should be in accordance with specs
33 Suspended Ceiling Systems Wide range of systems available to protect floors, beams and girdersFire resistance ratings published by ULRequire careful integration of ceiling tile, grid and suspension systemOpenings for light fixtures, air diffusers, etc. must be adequately limited and protected.Sometimes code requires individual structural element protection, thus suspended ceilings are not permitted.
34 Concrete and MasonryConcrete used to be common, but not highly efficient (weight and thermal conductivity)Concrete floor slabs are common for tops of flexural members.Concrete sometimes used to encase columnsfor architectural or structural purposes,or for protection from abrasion or other physical damage
36 Architecturally Exposed Structural Steel AESS: easthetic choiceSteel – Insulation– Steel skinGives appearance of steel surface but has protectionWater filled tubular columnsPatented in 1884, but not used until 1960 in the 64 story US Steel Building in PittsburghFlame shielded spandrel girdersStandard fire test is not representative of the exposure for exterior structural elements.Can only be used if code allows engineered solutions
38 Water Filled ColumnsColumns are interconnected with a water storage tank.In fire, water circulates by convection, keeping the steel temperature below the critical value of 450°C.This system has economical advantage when applied to buildings with more than 8 storeys.If the water flow is adequate, the resulting fire resistance time is virtually unlimited.In order to prevent freezing, potassium carbonate (K2CO3) is added to the water.Potassium nitrate is used as an inhibitor against corrosion.
41 Restrained and Unconstrained Major confusion from concept of Restrained and Unconstrained ratingsOnly in ASTM E119 and US codesNo other country uses thisPart of problem is max test size is 15’ x 18’ – not full scaleWhen testing problems arise:Floor slabs and roof decks are physically continuous over beams and girders, but this is too bigBeams join columns and girders in a number of different ways – can’t test them all
42 Restrained and Unconstrained ASTM E119 includes 2 test conditions: Restrained and UnrestrainedRestraint is against thermal expansionThis allows for thermal stresses from surrounding structureMost steel framing is tested as RestrainedUnrestrained:Single span and simply supported end spans of multiple baysOpen web steel joists or beams, supporting precise units or metal deckingWood construction
43 Temperature of exposed steel elements Rate of temperature change depends on mass and surface area.The weight to heated perimeter ratio is significantW/DW = weight per unit lengthD = inside perimeter of fire protection materialW/D = Thermal Size (lbs/ft/in)