A Beginner's Guide to ASCE 7-05

Chapter 6 - F_a: Flood Loads

© 2007, T. Bartlett Quimby

Section 6.6

Impact Loads

Last Revised: 11/04/2014

ASCE 7-05 5.4.4 states that "impact loads are those that result from debris, ice, and any object transported by floodwaters striking against buildings and structures, or parts thereof. Impact loads shall be determined using a rational approach as concentrated loads acting horizontally at the most critical location at or below the DFE."  The ASCE 7-05 commentary expands on this requirement.

The ASCE 7-05 commentary explains the current research on flood borne impact loads and recommends an equation for computing the force due to most debris impact.  The key variables are:

• Debris Weight, W:  The recommended weight is 1,000 lbs however, local knowledge may dictate differently.
• Velocity of the Object, V_b:  This is assumed to be the velocity of the water.  In the commentary it does suggest that this velocity could be lower for large objects in shallow water.
• Impact Duration, Dt:  This is the time that it takes for the object to come to rest and is a function of the dynamic properties of the impacted structure.  Considerable discussion is provided in the commentary about how to determine this key value.

The equation used is an adaptation of the impulse momentum equation and has other coefficients to modify for various conditions.

The resulting force is to be placed where it will have maximum effect at or below the DFE.  This recognizes that not all floods reach the DFE so the impact can occur anywhere up to that elevation. 

One thought that comes to mind, though not addressed in the commentary, is that since the velocity of the water changes with depth and the impact force is a function of the velocity, then it follows that the debris impact magnitude could be less for elevations below DFE.  However, it is conservative to ignore this relationship and use the force developed by impact in the design flood for all lesser conditions.

It should be noted that there is an excellent discussion on debris impact in the CCM.  The CCM uses the same impact momentum equation, without the modifying coefficients, as used in the ASCE 7-05 commentary.  The discussion in the CCM is valuable for understanding the nature of the event, however it appears that discussion is based on research that is older than some of the more recent research reviewed in the ASCE 7-05 commentary.