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Impact Load

Impact Load

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Impact Load

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  1. Impact Load • Live loads specified in codes do account for ordinary impact loads • When 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

  3. Crane Runway Loads • Structures supporting cranes: • Maximum wheel loads • Allowance for impact • Multiple cranes • Traction and braking forces • Use of crane stops • Cyclic loading / Fatigue • Crane live load is its fully rated capactity

  4. Cranes

  5. Crane Runway

  6. Crane load • Max vertical wheel load • Monorail, cab operated, remote operated • increased by 25% for impact • Pendant operated overhead • Increased by 10% for impact • Impact 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 rails • Acts normal to the rails • Distributed, as appropriate to stiffness of rail support • Bridge or monorail with hand-gearing • No need for lateral load increase

  8. Crane Stop Forces • Runway must be designed for stop forces • Velocity of crane at impact taken into account • Fatigue and serviceability concerns • AISC Design Guide 7 • AISE Standard No. 13

  9. Restraint Loads • Caused by changes in dimensions/geometry of structures due to • Behavior of material • Type of framing • Details of construction • e.g. • Foundation settlement • Temperature changes • Shrinkage restrained by adjoining structures

  10. Combined Loads • Loads may act simultaneously • Building codes specify various combinations that must be considered • Depends on whether allowable stress design (ASD) or Load and Resistance Factor Design (LRFD) is used • SEI/ASCE 7-02 provides guidance.

  11. Load Sources • D = dead load • L = live floor load, including impact • Lr = roof live load • S = roof snow load • R = rain load (initial rainwater or ice, exclusive of ponding) • W = wind load • E = earthquake load • T = 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 ] + D • 0.75 (W or 0.7E) + D • 0.75 [ L + (W or 0.7E) + (Lr or S or R) ] + D • 0.6D + W • 0.6D + 0.7E • Because E was calculated for LRFD it is reduced by 0.7 for ASD design.

  13. LFRD Design Loads (SEI/ASCE 7-02) • 1.4 D • 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 + 1.6W • 0.9D + E

  14. Fire Protection • International Building Code • International Code Council, Falls Church, VA • NFPA 5000, Building Construction and Safety Code • National Fire Protection Association, Quincy, MA • National Building Code of Canada • National Research Council of Canada, Ottawa, ON • Or local code

  15. Combustible/Non-combustible • Most fires are accidental or carelessness • Start small and require fuel and ventilation to grow • Noncombustibles (concrete, steel, brick) are not fuel • Combustibles (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 area • Fire severity is the duration of the fire, in hours of equivalent fire exposure • More modern approaches of fire load are expressed in terms of potential heat energy • Fire loading correlates well with fire severity

  17. Fire Loading • Reasonable estimate for conventional wood frame construction: • 7.5 – 10 lb/ft2 • Reasonable heavy timber estimate • 12.5 – 17.5 lb/ft2 • Consequently building codes limit permitted size (height and area) of combustible buildings more than non-combustible buildings. • BUT ventilation is an important factor as well.

  18. Occupancy Fire Loads and Severity

  19. Fire Resistance • Fire Resistance: Relative ability of construction assemblies to prevent spread of fire to adjacent spaces, and to avoid structural collapse • Fire resistance requirements are a function of occupancy and size (height and area) • Fire resistance is determined experimentally • ASTM E 119 • Uses “standard” fire exposure • Specified in terms of time of exposure

  20. Fire Resistance • Time during which an assembly • continues to prevent spread of fire, • does not exceed certain temperature limits, and • Sustains its structural loads without failure • Typically expressed in hours • Fire Resistance Directory, Underwriters Lab • Fire Resistant Ratings, American Insurance Services Gp. • Fire Resistant Design Manual, Gypsum Association

  21. Fireproof • No building is fireproof. • Avoid this term

  22. Effect of Temperature on Steel • In general, steel can hold 60% of yield strength at 1,000 F • Failures 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.

  23. Steel Strength vs Temperature

  24. Fire Protection Materials • Gypsum • As a plaster, applied over metal lathe or gypsum lathe • As wallboard, installed over cold-formed steel framing or furring • Effectiveness 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

  25. Gypsum • 3 kinds: • Regular, Type X, and proprietary • Type X: • Specially formulated cores for fire resistance. • Proprietary • Such as Typc C, even greater fire resistance • Type of wall board must be specified clearly. • Type and spacing of fasteners (and furring channels if applicable) should be in accordance with specs

  26. Spray Applied Materials • Most widely used • Lightweight mineral fiber and cementitious material • Sprayed onto beams, girders, columns, floor decks, roof decks • SFRM: Spray-applied Fire Resistive Materials • Generally proprietary formulations • Follow manufacturers recommendations! • Underwriter’s Laboratories specifies these

  27. SFRM • Cohesion/Adhesion are critical • Must be free of dirt, oil, loose scale • Generally light rust is OK • Paint can cause problems

  28. Suspended Ceiling Systems • Wide range of systems available to protect floors, beams and girders • Fire resistance ratings published by UL • Require careful integration of ceiling tile, grid and suspension system • Openings 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.

  29. Concrete and Masonry • Concrete 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 columns • for architectural or structural purposes, • or for protection from abrasion or other physical damage

  30. Architecturally Exposed Structural Steel • AESS: easthetic choice • Steel – Insulation– Steel skin • Gives appearance of steel surface but has protection • Water filled tubular columns • Patented in 1884, but not used until 1960 in the 64 story US Steel Building in Pittsburgh • Flame shielded spandrel girders • Standard fire test is not representative of the exposure for exterior structural elements. • Can only be used if code allows engineered solutions

  31. Water Filled Columns • Columns 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.

  32. Flame Shields Interior Exterior Painted girder

  33. Restrained and Unconstrained • Major confusion from concept of Restrained and Unconstrained ratings • Only in ASTM E119 and US codes • No other country uses this • Part of problem is max test size is 15’ x 18’ – not full scale • When testing problems arise: • Floor slabs and roof decks are physically continuous over beams and girders, but this is too big • Beams join columns and girders in a number of different ways – can’t test them all

  34. Restrained and Unconstrained • ASTM E119 includes 2 test conditions: Restrained and Unrestrained • Restraint is against thermal expansion • This allows for thermal stresses from surrounding structure • Most steel framing is tested as Restrained • Unrestrained: • Single span and simply supported end spans of multiple bays • Open web steel joists or beams, supporting precise units or metal decking • Wood construction

  35. Temperature of exposed steel elements • Rate of temperature change depends on mass and surface area. • The weight to heated perimeter ratio is significant • W/D • W = weight per unit length • D = inside perimeter of fire protection material • W/D = Thermal Size (lbs/ft/in)