Prepared by Marcia C. Belcher, PE

ASCE 7-05: Minimum Design Loads for

Buildings and Other Structures

ASCE 7-05 sets the basic principles of structural engineering that guide the selection of loads.

The Standard defines procedures for determining the load types and the probable combinations of those loads that should be considered in design.

All structures are designed to be safe and serviceable.

Safety and Serviceability are determined by the application of "Limit States".

A limit state is a mathematical statement that shows that a structure has sufficient capacity to resist a particular form of failure that is either safety or service related.

ASCE 7-05: Minimum Design Loads for Buildings and

Other Structures

Safety Limit State: Structure is strong enough to support all load events without causing harm to people and property.

Actual Force < Capacity

Serviceability Limit State: Any limit on the functionality of the structure that is not strength related.

◦ Ie. Deflection and vibration.◦ Neither are safety related, however excessive amounts of

either may cause the structure be useless for its intended purpose.

Actual Behavior < Limit on Behavior

Buildings are classified based on their occupancy.

Classification categories range from I to IV.

Classification reflects a progression of the seriousness of the consequence of failure from lowest hazard to human life (Category I) to the highest (Category IV) .

Table 1.1 from ASCE publication 7-05.

How design loads are calculated will depend on the occupancy category.

Things that will impact classification:

◦ Number of occupants and the period of exposure to extreme events

◦ A structure's status in relation to the community and its safety.

◦ The need for operation of a facility during and after an emergency

◦ The impact of release of hazardous materials as the result of structural failure

Many structural elements receive load from a floor or decking system.

The loads on the elements will be caused by an “effective area” and the “unit load”

ASCE 7-05 prescribes the “unit load” in pounds per square foot of surface.

Tributary Area

Tributary Area: Example of Parallel Framing System

• The shaded area represents the “effective area” or “tributary area” on beam CD.

•The load from the deck is transferred to CD via the joist system.

• The “uniform load” on CD is equal the “unit load” (psf) x “tributary width” (B).

A large open exhibit building with long

span truss girders.

Long Span Roof Truss Girders Mezzanine Area Awning Roof

Awning Roof with

Hip Beam

Load rests on roof deck

Roof deck transfers load

to supporting joists.Each joist supports

an area equal to its

span times half the

distance to the joist

on either side.

The joists transfer

their loads to the

supporting truss

girders.

Each truss girder supports an

area equal to its span times

half the distance to the girder

on either side.The truss girders

transfer their loads

to the supporting

piers and columns.

The pier supports half the

area supported by the truss

girder plus area from other

structural elements that it

supports.

Metal Deck/Slab System

Supports Floor Loads Above

Joists Support Floor Deck

Girders Support JoistsColumns Support Girders

The area tributary to a

joist equals the length of

the joist times the sum of

half the distance to each

adjacent joist.

The area tributary to a girder

equals the length of the

girder times the sum of half

the distance to each adjacent

girder.

The girders are not

single span so the

tributary area for the

columns cannot be

graphically determined

Deck carries load to edge

joist and wall.

Exterior joist carried load to

the supporting cantilever

beam ends

The load diagram for the

cantilever (excluding self wt)

consists of a single point

load at the end of the

cantilever.

The point load consists of

the reaction from the two

supported joists which

equals the tributary area (1/2

the cantilever span times the

spacing of the cantilevers)

times the pressure load on

the floor plus the self weight

of the joist.

For lateral pressures, the siding spans between the

horizontal girts (yet another fancy word for a

beam!)The girts support half the siding to the adjacent girts. This is the tributary area for

one girt.

The girts transfer their lateral load to the supporting beam-

columns.

The beam-columns do not support any roof load,

they are here to resist lateral forces that they receive from the girts.

They support an area that extends from locations

half way to the adjacent beam-columns on each

side and from floor to roof as shown.

The beam-columns transfer their lateral loads

equally to the roof and foundation.

Example: Tributary Area Load Calculations

For the framing plan shown, ALL of the floor load is being taken by the floor joist. Draw the FBD for beams GH, FE, AB and AD. The design load is 60 psf, L=30 ft and B=10ft.