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Re: Lateral Load Due to Foot-Traffic[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "Seaint@Seaint. Org" <seaint(--nospam--at)seaint.org>
- Subject: Re: Lateral Load Due to Foot-Traffic
- From: "S. Gordin" <mailbox(--nospam--at)sgeconsulting.com>
- Date: Tue, 8 Nov 2005 22:39:03 -0800
Looks like I am going be alone with my opinion, but here it is. I think there seems to be three scenarios of for the loading of your stairs; hydraulic analogy does not seem to be very applicable to either of those.
(1) Few people moving slowly or running. This scenario does seem to require any special considerations, and, in spite of possible dynamic loading, can be successfully accommodated by an adequate "static" design of the stairs/railings (for example, for forces per www.crowdsafe.com/FruinCauses.pdf). The design lateral force on the stair structure can be conservatively (IMO) modeled as follows: number of people that fits the width of the stairs running at, say, 10 mph one flight and simultaneously hitting the railing at its 90-degree corner. I think that for a 4-ft wide stair, a 1,000-lb lateral force will be on a quite conservative side - compare to the 12.5 psf wind acting on a 16'x4' projected area and yielding 800 lb of force.
(2) Crowd moving slowly. This type of loading does not seem to be able to impose any dynamic (asymmetrical) pressures on the stairs and railings. The pressure from the crowds will be relatively uniform and, in the absence of proper crowd control design, quite substantial (see the article mentioned above). The pressure will definitely increase at a 90-degree "elbow", but it still will be a static pressure affecting the railing rather than the lateral-force-resisting system of the stairs.
(3) Crowd moving fast. This loading is the most dangerous one, but not for the stair structure as a whole. The worst and primary effect of this scenario is the inevitable tripping and crushing of people on the stairs. From the structural perspective, that should imperatively and quickly stop any dynamic components of the crowd pressure, and impose the static pressures described in (2).
IMO, scenarios (2) and (3) cannot be resolved by structural engineering means; in fact, the "stronger" is the railing, the more hazardous is the crowd pressure. It's all architecture and crowd control.
Actually, your concerns may be justified by the fourth scenario: an organized group of people moving, and/or pushing, and/or leaning in a synchronized manner - in other words, intentionally rocking the structure. I hope you do not have to design for that; however, if you do, its still a purely and easily modeled dynamic problem with no hydraulic undercurrents. So, we are back to a modified scenario (1).
I hope we are not overcomplicating the issue. Indeed, does anybody have any info on the stairs failing due to the dynamic crowd forces or quasi-hydraulic asymmetrical pressures?
Steve Gordin SE
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