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How to determine deflection of open web steel joists in-situ

How to determine deflection of open web steel joists in-situ

How to determine deflection of open web steel joists in-situ

(OP)
I have a project where existing conform deck and concrete slab span over existing metal joists. The owner wants to have a deflection test after shelving is installed. I was thinking measure the current deflection with a high precision laser at 1/3 or 1/4 points as well as at the center. Document that the baseline is the same for all measurements, then come back after loading and measure defelction again.

This is not to verify that the load can be supported, that as already been deteremined through analysis. This owner just wants to have as much documentation as possible on the condition of the structure before and after loading.

RE: How to determine deflection of open web steel joists in-situ

some thoughts:
zip level would work
good laser would work.
i'd consider doing the following:
1. calc expected deflection change, in order to get an idea of what to expect and thereby determine the accuracy of the laser you want. Home Depot varieties are likely not accurate enough.
2. considering the accuracy you want, precisely mark on the floor the positions at which you take the initial measurements, so as to be able to obtain comparable measurements post subsequent deflection.... helps you dodge issues with high and low spots on your base below your joists.
3. develop laser technique to make sure your aiming true vertical. I don't think this would be needed with zip level.
4. depending again on accuracies, i'd consider getting lots more data points, which takes not very much time, say at each bottom chord panel point.
5. consider that supporting girders will deflect too, if that is of interest to you.
6. if you're completely obsessive like me, you could get some relative changes between the top of your deck and the structure above, the increase of which would correspond to the decrease in elevation below the deck.
have fun!

RE: How to determine deflection of open web steel joists in-situ

Assuming the supports for the joists will not displace from the load, simply string "piano wire" from support to support. Place a weight on at least one free end of the wire to allow for temperature compensation. Put a mirror on the joist at midspan and quarter points and attach a precision ruler to each mirror. The reflection of the wire on the mirror will give you an accurate measurement. Check before and after and you will have it. It can stay in place and you don't have to worry about having a laser line in the same place when you bring it back out to check later. Simple, inexpensive and accurate.

RE: How to determine deflection of open web steel joists in-situ

I bet ron gets done twice as much as I do in half the time bow

RE: How to determine deflection of open web steel joists in-situ

Ron, you left off the part about getting done early and going to have a beer or two.

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RE: How to determine deflection of open web steel joists in-situ

How much deflection do you predict and what is the owners expectation? We have a 3D laser scanner, so we would scan before and after. 9min per scan, plus time to setup and 20min to import the scan data. That said, if your trying to find 1/4" or so, Ron's method is golden.

RE: How to determine deflection of open web steel joists in-situ

(OP)
Thanks for your input. I will have to do this on a few joists so I may go with the first option. I'll try and determine expected deflections, I didn't think about that.

RE: How to determine deflection of open web steel joists in-situ

brad805,
i'm drinking 805 beer atm,
what kind of scanner, and what kind of accuracy? I've been looking at these.....

RE: How to determine deflection of open web steel joists in-situ

Triangled, we went with a Topcon GLS2000, but the Faro 130 or 330 are more popular. Leica is another big player too, but they are quite a bit more expensive. The scanning is quite easy, but how one processes the data and converts it into something useful can be a challenge. We rented a scanner several times, and I think that is a good place to start. The accuracy is not a simple question to answer. Generally, most report 4 - 10mm, but it depends on the reflective quality of the surface, distance, angles, device settings, and so on. For us, the benefit has been the ability to collect data that clients would not normally pay us to collect. This is a new market we are developing for plant retrofit work. Enjoy the beer.

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