Impact Load

Report
 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
Type of
member
Source of Impact
Percent
increase
Supporting
Elevators and elevator machinery
100
Supporting
Light machines, shaft, or motor driven
20
Supporting
Reciprocating machines or power-driven
units
50
Hangers
Floors or balconies
33
 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
 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
 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
 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
 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
 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.
 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
 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.
 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
 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
 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
 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
 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.
8
7
Fire Severity (hrs)
6
5
4
3
2
1
0
0
10
20
30
40
Fire Load (lbs/ft2)
50
60
70
Type of Occupancy
Fire Load (lb/ft2)
Fire Severity (hrs)
Assembly
5-10
0.5 – 1
Business
5-10
0.5-1
Educational
5-10
0.5-1
Hazardous
Variable
Variable
Low hazard
0-10
0-1
Moderate hazard
10-25
1-2.5
Institutional
5-10
0.5-1
Mercantile
10-20
1-2
Residential
5-10
0.5-1
Low hazard
0-10
0-1
Moderate hazard
10-30
1-3
Industrial
Storage
 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
 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
 No building is fireproof.
 Avoid this term
 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.
1
% Yield Strength
0.8
0.6
0.4
0.2
0
0
500
1000
1500
Temperature (F)
2000
2500
 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
 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
 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
 Cohesion/Adhesion are critical
 Must be free of dirt, oil, loose scale
 Generally light rust is OK
 Paint can cause problems
 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.
 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
 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
 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.
Interior
Exterior
Painted girder
 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
 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
 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)
4
3.5
Fire Resistance (hrs)
3
2.5
2
1/2"
5/8"
1.5
1"
1 1/4"
1
1 1/2"
1 7/8"
0.5
2"
2 1/2"
0
0.5
1.5
2.5
W/D (lb/ft/in)
3.5

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