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Impact Load

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. Minimum % increase in live load on structural m embers due to i mpact. Crane Runway Loads.

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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)

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