Allowable Stress Increase for Impact Loading?? 1

[jonnywalker]

I am designing a steel beam that will have an impact load. After impact, the load will almost instantaneously go to zero. I was told that there is an allowable stress increase of 1.6 - 2.0 for these short duration loads. I cannot find anything that supports this increase. Ive looked through the AISC (13th ED.), ASCE 7, and the California Building Code. There are allowable stress increases for impact loads in both the NDS and the bridge manual, but one is wood and I think the other is for concrete.

I could not find anything online either. I was told that AISC 9th ED. had a section that referenced this, but I dont have a copy of that, and if it is no longer in the 13th ED should I still use it??

If anyone knows the reference to a current code or an opinion on whether I should bump the stress for the allowable stress it would help me out a ton! Thanks for your time.

 

[EngineeringEric]

How long is the duration of the impact? you can analyze the impact load as an impulse load, possible that it could count as a lower static load than the F=ma force you may be dealing with.

 

[SlideRuleEra]

jonnywalker - AISC 9th ED. did allow a range of increases for impact, depending on the use of the structure. Now, the only increase appears to be for elevator structures, and that is not by AISC but reference to ASCE 7 and ASME A17.1. See the second page of the "Steel Interchange" section of AISC's July 2011, issue of "Modern Steel":

http://www.modernsteel.com/Uploads/Issues/July_2011/072011_July11_SI_web.pdf

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[NOLAENG]

After looking at section A5.2 in the 9th edition, I don't believe it says you can take an increase in stress for impact loads.

"Allowable stresses may be increased 1/3 above the values otherwise provided when produced by wind or seismic loading, acting alone or in combination with the design dead and live loads, provided the required section computed on this basis is not less than that required for the design dead and live load and impact (if any) computed without the 1/3 stress increase."

 

[nutte]

SlideRuleEra, the increase in the 9th edition was on the load, not the allowable stress. For certain impact loads, they told you how much to increase the magnitude of that load to account for impact.

Smhetrick, that section was removed in a supplement to the 9th edition specification.

 

[SlideRuleEra]

Thanks, nutte. I went back and reread both this question and AISC 9th Ed. You are absolutely right.

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