Design load to check lateral stiffness of walkway
Design load to check lateral stiffness of walkway
(OP)
I'm designing a walkway that spans 18 ft with a 100 psf load. I want to ensure that the walkway is stiff and does not sway too much side to side. I've seen and walked on too many long span walkways that have significant lateral movement. Although it's structurally safe, the movement makes people nervous. What load should be applied in the lateral direction to check the stiffness in this direction?






RE: Design load to check lateral stiffness of walkway
RE: Design load to check lateral stiffness of walkway
RE: Design load to check lateral stiffness of walkway
RE: Design load to check lateral stiffness of walkway
RE: Design load to check lateral stiffness of walkway
What loads do you think will generate lateral movement? Perhaps a percentage of the live load. Maybe 5%.
RE: Design load to check lateral stiffness of walkway
RE: Design load to check lateral stiffness of walkway
RE: Design load to check lateral stiffness of walkway
RE: Design load to check lateral stiffness of walkway
RE: Design load to check lateral stiffness of walkway
If you've noted lateral displacement to be an issue then what you really want to do is limit lateral displacement. You need a lateral deflection limit, not more load.
I'm sure there are several articles written on the subject as there are deflections limits for beams in flooring as well as limits on vibration characteristics.
Another example is that highway bridges with pedestrian traffic are limited to L/1000 in deflection.
Look to the codes for examples of where deflection limitations are provided and see how they can be applied in your case.
Regards,

Qshake
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RE: Design load to check lateral stiffness of walkway
"At the end of the day, you could apply several imaginary loads and still not ensure you've met the requirements that you've specified for yourself."
BINGO!!! This is what I was hoping to avoid. I don't guess there is a known load that can be applied to check whether or not the system has enough lateral stiffness.
Another thought I had was that if I have two main channels oriented like so ][ with a 3 foot spacing between them and lateral members (say channels) between these two channels that are the same depth as the main channels (spaced 3-4 feet apart), wouldn't this form one big beam? If so, the 3 foot depth would make this walkway have sufficient lateral stiffness.
RE: Design load to check lateral stiffness of walkway
You need to consider the possibility of dynamic response of the bridge to pedestrian-induced excitation.
Footfall frequency of pedestrians is typically in the range of 1.5 to 4 Hz, depending on their walking speed. Due to the mechanics of body balance, pedestrians also create lateral forces at half the footfall frequency (in the range from 0.75 to 2 Hz). If any of your significant mode shapes have natural frequencies anywhere in this range, you need to seriously consider a dynamic check. In particular, any walkway with a lateral sway frequency less than about 1.3 Hz is possibly prone to “synchronous lateral excitation”.
A key factor is whether the bridge is potentially subject to crowd loading. Loosely scattered pedestrians tend to walk with random, uncorrelated steps (as long as we are not dealing with a squad of marching soldiers or similar). However, if the crowd density gets high enough, but not so high that movement effectively ceases, people tend to subconsciously fall into step with each other, to avoid tripping over each other’s feet. (This phenomenon has been called “syncopated shuffle”.)
If the frequency of footfall closely matches a natural frequency of the bridge, especially a lateral mode shape, the bridge will start to sway in response, and then the frequency of the pedestrians’ footfall will tend to “lock in” to the bridge lateral sway frequency, as people subconsciously match their footfall to the movement of the bridge. This phenomenon can lead to dramatic swaying of the structure, as was exhibited on the opening day of the London Millennium Bridge in June 2000, for example.
The subject is very complex, but the magnitude of the lateral force induced by a walking pedestrian is typically in the order of 4% to 10% of the person’s weight, depending on whether the bridge is “static” or swaying noticeably in response to the pedestrian excitation.
Here are a couple of papers to get you started:
http://go.rwdi.com/technotes/t15.pdf
www.a
Hope this helps!
RE: Design load to check lateral stiffness of walkway
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RE: Design load to check lateral stiffness of walkway
In order for the channels to act as a singular 3ft member, you'd either have to provide diagonal bracing or the strut to channel connection would need to transfer moment. In this circumstance, you're bettor off using WF's and letting the erector field weld diagonal L's to the underside of the top flange. This method of stiffening a walkway should not raise the price very much. fabrication cost is not increased, and the erector is given some leeway in how he puts it up. Of course there could be asthetic reasons for not doing it this way.
If the struts are the same depth as the beams, the eccentricity of the L's won't shouldn't much of a problem.