A Beginner's Guide to ASCE 7-05

Chapter 1 - General Information

© 2008, T. Bartlett Quimby

Section 1.2

Basic Requirements

Last Revised: 03/04/2015

ASCE 7 Section 1.3 sets for the basic principles of structural engineering that guide the selection of loads.

If a single statement can be made that defines the role of structural design, it would probably be;

All structures must be designed to be safe and serviceable.

Safety and Serviceability are determined by the application of "Limit States".  A limit state is generally a mathematical statement that shows that a structure has sufficient capacity to resist a particular form of failure that is either safety or serviceability related.  Limit states are generally expressed as inequalities.

Be sure to read the ASCE 7 commentary on this (and all sections).  The explanations there are very well written.  This discussion is intended to supplement the commentary, not replace it.

Safety Limit States

For structural engineering, safety is synonymous with strength.  A safe structure is a structure that is strong enough to support all load events without causing harm to people first and property second.  Examples of safety limits include limits on capacity based on material yielding and/or fracture, and both local and general structural stability.

A typical safety limit state takes the form of:

Actual Force < Capacity

The capacity of the structure or structural element under consideration is not the purview of this Standard.  Computation of capacity is generally specified by industry wide organizations such as the American Institute of Steel Construction, the American Concrete Institute, and others.

The actual force in a member or structure is a function of the applied loads is determined using principles of structural analysis.  The Standard is focused on determining the loads to be used in determining the required actual forces to be compared against the capacities.

Serviceability Limit States

Serviceability limit states include any limit on the functionality of the structure that is not strength related.  The most common structural serviceability limit state are deflection and vibration.  Neither deflection nor vibration are safety related, however excessive amounts of either may cause the structure to not be usable for its intended purpose. 

A typical serviceability limit state takes the form of:

Actual Behavior < Limit on Behavior

As with the safety limit state, it is not within the purview of the standard to define the limit on behavior.  Indeed, most of the material specific specifications and standards give little or no guidance on these limits.  The criteria for serviceability is generally project dependent.  For example, a flexible floor system may be very desirable for a aerobics fitness center but not acceptable for a micro surgery center.

Also, similar to the safety limit states, the actual behavior is determined using the applied loads and principles of structural analysis.

Other Considerations

An interesting feature of ASCE 7 section 1.3 is that it highlights some basic considerations that are occasionally over looked in structural design.

The first item highlighted is self-straining forces.  While the Standard does not provide a section for quantifying these forces, they must be considered using good structural analysis procedures.  For continuous and other restrained structures the effects of temperature variation, material shrinkage/swelling due to moisture change, creep under sustained loading, and "other similar effects" must be accounted for.  These actions may result in large internal forces.  The standard recognizes the safety consideration of these loads but leaves it to the design engineer to determine the rational means for determining these forces using recognized procedures and principles.

Since the demand side of a typical limit state inequality involves both loads and structural analysis, the Standard highlights that analysis considerations that are a part of the analysis process.  Basic Statics, material mechanics, and stability all must be a part of an acceptable analysis procedure.

By sad experience it has been found that certain principles of stability and statics have been neglected over the years when it comes to resisting the lateral forces of wind and earthquake.  ASCE 7 section 1.3.5 makes particular emphasis of these principles.  Overturning, sliding, and complete load paths are part of any complete structural analysis and must not be overlooked in the design of any structure.  In particular the combination of other loads that counteract or add to the overturning and sliding effects is discussed.