Analyzing Existing Steel Structure 2

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I’ve been asked to furnish a proposal for analyzing capacity of an existing mezzanine supported by a simple steel structure (HSS truss joists/girders, HSS columns, corrugated decking, etc) in an industrial unit. I primarily repair existing structures, but I don’t much analyze them for others / provide them with design loads of such; so, I had a few questions / concerns that I wanted to know how others have dealt with:

1. In order to determine steel properties I will take coupons. My question is how many? I figured at least one in every type of structural element though am thinking I may want more to make sure the variance per element is within a tolerable range.

2. I have yet to visit the site but from pictures it looks like the structure is well built, though the connections appear a bit suspect. For example, the supporting HSS columns (appear to be 4x4 of unknown thickness with a 10x10 BP) have 4 threaded rod anchors that appear to be 1/4" or LESS. Now, the structure is made up of interconnecting simply supported elements so technically the capacity shouldn't be governed by these anchors...but for comparison I specify 1/2" anchor rods for garden variety steel guard rails on accessibility ramps. Compared to the column it looks like the anchors came from a kid's playset. Add to that the anchors will be of unknown embedment with unknown embedding methods. I'm concerned of you know...things (forklift hits the structure while fully loaded, someone impacts the structure by dropping a pallet rapidly that causes deflection leading to P-Delta that destroys these flimsy anchors, etc). How do I evaluate the capacity of the structure in light of this?

3. How do I reconcile the theoretical capacity of the structure vs the installed structure, which has connections that I couldn't verify / welds I didn't inspect prior to paint, etc? I imagine if I provide a theoretical capacity with a caveat that it is based on a certain weld condition (that cannot be checked at present or at least not for a reasonable sum of money) that I would be liable in the event that something happens upon loading to the theoretical limit. Do I just calculate a theoretical capacity and add a comfortable margin of safety to account for such things?

4. Other things to consider from those that provide reports on such things?

Thanks

 

[dik]

First questions... are there existing drawings and specifications for the structure? Does the municipality/town or whatever have drawings? Any permit drawings? or even permits?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[phamENG]

1. A good idea. To figure out how many/where, I'd start by doing an analysis and Identifying the most critical areas. Do you know when it was erected? If so, you can determine the most likely steel grades based on age and start there. If they work easily, then I'd just take one or two to confirm (if that - if they pass all common steel of that era with an acceptable margin I wouldn't take any coupons). If the design is questionable based on your assumption, take enough that you feel comfortable that you know just how strong the steel is where you need it most.

2. I wouldn't worry too much about the base if it works as a pin. I'd discuss your operational hazards concerns with the owner/operator. Either they have other means of minimizing the risk (no forklifts under the mezzanine?) or you can convince them of it as well. Then either: shore up and demo a typical column to find out just what's there OR put in bollards.

3. For the welds, again I'd see what the analysis says. If demand is low enough, I'd let it go. At critical welds, get out with a wire brush and remove some paint and measure the fillet leg. There will always be some level of unknown (unless you tear and down and test every piece, but then where will you be?), so you have to look at things like past usage. If it has a theoretical capacity of 1000psf but it has been used to store cotton candy, I wouldn't give them the whole 1000psf - it's never really been tested. If it checks out to 100psf but they've been storing pallets of bricks on it for 50 years, I'd probably give them the full 100psf. (These are obviously extremes - you'll probably be somewhere in between.)

4. Rather than drilling holes in everything, consider getting an ultrasonic testing tool. They aren't cheap, but can come in handy when you need a steel thickness but can't drill into it. This may not be a problem for you, though, since you are one of those rare contractor and engineer types and you're probably equipped for it.

 

[kipfoot]

1. What phamENG said. From what you’ve described, I doubt that the steel capacity will be your weak link

2. What phamENg said.

3. Again, I agree with phamEng. You may be find that it’s a situation where you can give them, for example, 50 psf, but if they reinforce these connections, x, y and z, you get 100 psf.

4. Be sure to include a couple site visits in your proposal. The first time you’ll get the sizes, dimensions, and basic connections. Then, after you do an analysis you’ll probably want to go back with a good framing plan and look at a few issues more closely.

Also be clear about what testing may need to be done and whether they’ll need an outside testing firm to do the tests. I recently had a prospective client say, “oh, I though you would do the excavation to expose the footings.”. Um, no.

 

[phamENG]

I recently had a prospective client say, “oh, I though you would do the excavation to expose the footings.”. Um, no.

Ha. Yeah. For residential stuff I'll occasionally put in that I'll attempt to expose a small portion of a footing for verification, but if it's more than a foot deep I usually say they have to get somebody to expose it. If they complain, I explain that on an hourly basis they can hire a laborer to do it for 1/10th (or less) of what they'll pay me. And lets face it, my shoveling muscles have atrophied a bit sitting at this desk - the laborer can probably do it twice as fast.

 

[Enable]

dik: The client inherited the structure from the previous owner and as far as I know there are no records of the build. However, I will request permit / drawing records from the city and see what they have on file. I'm doubtful but I might just get lucky! I will also inquire as to what the previous owner stored up there as I am hoping the client has some idea(thanks to phamENG for providing the idea that previous use acts as an approximate load test that can be used to corroborate a theoretical capacity).

phamEng / kipfoot: Very much appreciate the insight. I will qualify with limitations based on visual review, etc but I just wanted to know if there is anything typical that I was missing since this is not what I do day in / out. Sounds like not but to reinforce I should a) be conservative and b) make sure I locate critical elements and look at them closely (surely should go without saying but sometimes easy to forget so thanks for highlighting).

And you are right about the contracting bit. The nice thing is I have all the fancy expensive toys and if I don't I can always find a reason to buy them for a job!

Here is an example I am fond of: I had a job about 3 years back where I needed to get to the backside of an existing condo with some moderately heavy material (concrete bags, short stubby steel beams, rebar, etc) but I didn't want to tear things up too much (hell of a long way to re-sod for how much the job cost). So instead of dropping my skidsteer I justified the purchase of a used 4x4 and new aluminum trailer (all in $3,500 CND or so). Probably unnecessary but I've always wanted a 4x4 on a jobsite lol and the best part was the 4x4 had a hitch to haul material in the trailer on-site but the 4x4 itself fit into the trailer and we used that to transport it from the shop! 3 years later the setup is still humming.