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
Impact Load
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
Minimum % increase in live load on structural members due to impact
Structures supporting cranes:Maximum wheel loadsAllowance for impactMultiple cranesTraction and braking forcesUse of crane stopsCyclic loading / Fatigue
Crane live load is its fully rated capactity
Crane Runway Loads
Cranes
Crane Runway
Max vertical wheel load Monorail, cab operated, remote operated
increased by 25% for impactPendant operated overhead
Increased by 10% for impact
Impact increases do not have to be applied to supporting columns, only runway
Crane load
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-gearing
No need for lateral load increase
Crane Lateral Loads
Runway must be designed for stop forcesVelocity of crane at impact taken into accountFatigue and serviceability concerns
AISC Design Guide 7AISE Standard No. 13
Crane Stop Forces
Caused by changes in dimensions/geometry of structures due to Behavior of materialType of framingDetails of construction
e.g.Foundation settlementTemperature changesShrinkage restrained by adjoining structures
Restraint Loads
Loads may act simultaneouslyBuilding codes specify various combinations
that must be consideredDepends on whether allowable stress design
(ASD) or Load and Resistance Factor Design (LRFD) is used
SEI/ASCE 7-02 provides guidance.
Combined Loads
D = dead loadL = live floor load, including impactLr = roof live loadS = roof snow loadR = rain load (initial rainwater or ice, exclusive of
ponding)
W = wind loadE = earthquake loadT = restraint load
Load Sources
DD + L + TD + (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 + 0.7E
Because E was calculated for LRFD it is reduced by 0.7 for ASD design.
ASD Loads (SEI/ASCE 7-02)
1.4 D1.2(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
LFRD Design Loads (SEI/ASCE 7-02)
International Building CodeInternational Code Council, Falls Church, VA
NFPA 5000, Building Construction and Safety CodeNational Fire Protection Association, Quincy,
MANational Building Code of Canada
National Research Council of Canada, Ottawa, ON
Or local code
Fire Protection
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
Combustible/Non-combustible
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
Fire Loading and Fire Severity
Reasonable estimate for conventional wood frame construction:7.5 – 10 lb/ft2
Reasonable heavy timber estimate12.5 – 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.
Fire Loading
0 10 20 30 40 50 60 700
1
2
3
4
5
6
7
8
Fire Load (lbs/ft2)
Fir
e S
eve
rity
(h
rs)
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
Industrial
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
Storage
Low hazard 0-10 0-1
Moderate hazard 10-30 1-3
Occupancy Fire Loads and Severity
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 119Uses “standard” fire exposureSpecified in terms of time of exposure
Fire Resistance
Time during which an assembly continues to prevent spread of fire, does not exceed certain temperature limits,
andSustains its structural loads without failure
Typically expressed in hours
Fire Resistance Directory, Underwriters LabFire Resistant Ratings, American Insurance
Services Gp.Fire Resistant Design Manual, Gypsum
Association
Fire Resistance
No building is fireproof.Avoid this term
Fireproof
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.
Effect of Temperature on Steel
Steel Strength vs Temperature
0 500 1000 1500 2000 25000
0.2
0.4
0.6
0.8
1
Temperature (F)
% Y
ield
Str
en
gth
GypsumAs a plaster, applied over metal lathe or
gypsum latheAs 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
Fire Protection Materials
3 kinds:Regular, Type X, and proprietary
Type X:Specially formulated cores for fire resistance.
ProprietarySuch 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
Gypsum
Most widely usedLightweight mineral fiber and cementitious
materialSprayed onto beams, girders, columns, floor
decks, roof decksSFRM: Spray-applied Fire Resistive Materials
Generally proprietary formulationsFollow manufacturers recommendations!Underwriter’s Laboratories specifies these
Spray Applied Materials
Cohesion/Adhesion are criticalMust be free of dirt, oil, loose scale
Generally light rust is OKPaint can cause problems
SFRM
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.
Suspended Ceiling Systems
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 columnsfor architectural or structural purposes, or for protection from abrasion or other
physical damage
Concrete and Masonry
AESS: easthetic choiceSteel – Insulation– Steel skin
Gives appearance of steel surface but has protection
Water filled tubular columnsPatented in 1884, but not used until 1960 in
the 64 story US Steel Building in PittsburghFlame 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
Architecturally Exposed Structural Steel
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.
Water Filled Columns
Flame Shields
Interior
ExteriorPainted girder
Major confusion from concept of Restrained and Unconstrained ratings
Only 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 big
Beams join columns and girders in a number of different ways – can’t test them all
Restrained and Unconstrained
ASTM E119 includes 2 test conditions: Restrained and Unrestrained
Restraint 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 bays
Open web steel joists or beams, supporting precise units or metal decking
Wood construction
Restrained and Unconstrained
Rate of temperature change depends on mass and surface area.
The weight to heated perimeter ratio is significant
W/D W = weight per unit lengthD = inside perimeter of fire protection material
W/D = Thermal Size (lbs/ft/in)
Temperature of exposed steel elements
0.5 1 1.5 2 2.5 3 3.50
0.5
1
1.5
2
2.5
3
3.5
4
1/2"
5/8"
1"
1 1/4"
1 1/2"
1 7/8"
2"
2 1/2"
W/D (lb/ft/in)
Fir
e R
esi
stan
ce (
hrs
)