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

)