Anchor Loads Supporting a Debris Shield Net

[KF]

I am designing a debris shield for a bridge demo. We're using a net system which anchors into the side of either abutment. There will be three evenly spacing anchors on each side of the abutment. I am trying to determine what the reactions will be at each anchor bolt. Because the net is not a rigid structure, I am not sure how to distribute the loads. Any insight would be great! Thank you!

 

[Lomarandil]

What sort of loads are you considering? And is the net configuration (esp. connections to these anchors) relatively redundant, or not?

uniform concrete debris or similar -- I'd go with a tributary area type distribution, with generous tributary areas to account for imperfect installation, continuity, or uneven loading. (As a starting point, I'd design the edge anchors for 35% of the net width each, and the middle anchor for 65%)

personnel loads -- Post the heck out of it (e.g. 4 workers maximum), then assume that all 4 of those workers will at some point gather to take a look at one of your anchors. The "Hey, boss, this doesn't look right" effect.

wind loads -- really depends on your net system.. but potentially all lateral load will go into one anchor per abutment.

 

[WARose]

Because the net is not a rigid structure, I am not sure how to distribute the loads.

You probably are going to have to make some judgment calls (with a healthy safety factor) to distribute the loads via a tributary area method. One critical thing will be angle the net will make with anchor: you may wind up with a lot of shear. (The forces developed can be figured like a suspended cable system in most structural analysis texts.)

By the way, I'd have a dialog with the contractor as to how much debris he is expecting and what you expect the max to be (and document this in the design drawings). I've heard horror stories about a lot more stuff winding up in these nets than anyone was expecting.

 

[dhengr]

KF:
I certainly would want to know, from the net manufacturer, what the net breaking strengths are under various load conditions, and also what the breaking strengths of the net anchor points on the net are. Then, I would work back from there with some FoS on the various breaking strengths. What loads will it take, and how do these compare with reasonable applied loads?

 

[bridgebuster]

Sounds like you're using something on the order of a SafeSpan System Link, although SafeSpan uses metal decking not nets supported by cables. No, I am not an employee of SafeSpan but I have checked their submittals for several projects. Essentially, you're designing a cable and cable anchorages, which will have tension and shear. Depending on your span length you'll probably need vertical cables to take some of the load from the main cable. Typically for cables the FS is 4.

Here's where it can get tricky, you'll need to look at different cable angles to determine your maximum cable reactions. The cable deflection is going to be a function of how much debris is in the net. You can model your system in STAAD or similar program to eliminate some of the repetitiveness of the calculations.

Out of curiosity: What's below the span being demo'ed? I've never seen an owner allow catch nets in lieu of a rigid shield. One disadvantage of nets is that you can't have workers on them. It's not easy walking on netting - been there, done that, don't want to do it again.