A Beginner's Guide to ASCE 7-10

Chapter 1 - General Information

© 2012, T. Bartlett Quimby

BGASCE7-10 Section 1.3

General Structural Integrity

Last Revised: 11/04/2014

ASCE 7-10 Section 1.4 is written to require structures to be designed such that general failure (collapse) of a structure does not occur as the result of a local failure.

The section explicitly states the need for continuous load paths for both gravity and lateral force-resisting systems.  The section goes into some detail to make sure that all connections are secure regardless of what analysis says.

New to this section in ASCE 7-10 is the requirement of "notional loads" and their combination with other loads.  These loads are not dead, live, wind, sesimic, etc. but are their own load type and as such are combined as separate loads as specified in section 1.4.1.  A quick look at dictionary definitions for 'notion' say something like "a general understanding; vague or imperfect conception or idea of something: a notion of how something should be done." These notional loads fit the definition well, as they are loads which don't show up in a classical analysis, but we know there should be some strength provided there none-the-less.

For example, analysis of a simply supported beam not subjected to loads parallel to its axis might indicate there is not a reaction parallel the member axis at the supports and the designer may be tempted to not provide connection strength in that direction or, it strength is provided, there is no other recommendations for how much strength must be provided.  In this case, ASCE 7-10 section 1.4.4 requires that connection to provide strength to resist at least 5% of the unfactored dead plus live load which is imposed on the supporting member in another direction. This additional load is a "notional load", N. This ensures that the connection will not separate under incidental loading.

Additional sections look at load path connections, basic lateral forces regardless of the lack of seismic and/or wind exposure in some cases, securing walls and dealing with extra ordinary loads and events (progressive collapse, for example).

Section 1.4.6 is deceptively short.  There is a substantial history and body of research behind this concept.  In fact, the commentary devotes several pages to the discussion of this one paragraph.  The commentary is well written and must be read to understand what ASCE 7 section 1.4 really means.

Of particular concern is structural failure as the result of "accidents, misuse, and sabotage".  These events are very difficult to anticipate and quantify.  The commentary pointedly emphasizes that it does not "establish specific events to be considered during design".  There is always a risk of accidents, misuse, and sabotage, however it is the nature of these events that it is virtually impossible to anticipate all possible means by which they occur nor the level of risk to which a given structure will be exposed over its life time.  The best that we can do is learn from the past and take "reasonable" measures to mitigate the risk.  Design standards will be continually updated to codify procedures to reduce risk from failures that have been witnessed in the past.  There will always be cases of unanticipated events causing failure in structures.

The commentary classifies collapse into two categories and gives an example of each.  The two classifications are General Collapse and Limited Local Collapse.  A goal of good design practice should be to limit the risk of general collapse as a result of the failure of a subset of critical structural elements.

Two particular examples are used by the commentary to illustrate general collapse.  You should read the commentary for an excellent summary of the structural issues.

Ronan Point:  This structural failure was dramatic and led to extensive dialog concerning general collapse as the result of an accident.  The particular type of collapse has been termed as "progressive collapse".  A quick web search will turn up a number of references that discuss this failure.  The classic image of the collapse is shown on a number of the sites, but can be easily found on at Wikipedia.  Click here to check it out.  Clicking on the image provides a larger view.

Alfred P. Murrah Federal Building:  This collapse was the result of a sabotage.  The disintegration of some key columns magnified the effect of the blast.  Again, Wikipedia has a some good image of the general failure.  Click here to check it out.

The commentary provides a list of factors that have increase the risk of damage propagation in modern structures.  Check it out.

The basic principles that should be considered in design include redundancy, ductility, and alternate load paths. The section of the commentary on guidelines for the provision of general structural integrity (section C1.4) is must reading for all structural design engineers!  You may need to work with your architectural clients to implement some of these practices.