Age of steel building to rule out A36? Composite construction? 2

[ThorenO2]

Hello,

I'm analyzing a 120,000sf factory floor, 2VLI 8" total thickness, that appears to be built for WWIII for a new piece of machinery. It was constructed as an addition with W24x55s @ 6'-3" o.c. x 30' w/ W33 Girders spanning 25'. The steel is fireproofed and from historic Google Satellite images, it wasn't there in '94, but appeared in '02, so a maximum age of 27. No useful data points from the owner, auditor, or municipality. Are there any clues that would indicate what grade of steel should be used in the analysis, either 50 ksi A992/A572 or 36 ksi A36 for a building of this age? Deflection typically governs, except in one instance at a since infilled floor opening where there was previously a gigantic concrete pylon.

My second question is, what is the likelihood that the floor was designed as composite and would smaller members used to frame openings also be composite (running in same direction)? Are there rules of thumb where you only design the main floor members as composite or is there a rule of thumb to assume it is not composite at all, since you wouldn't have the headed stud properties anyway? Thank you in advance for any input,

 

[canwesteng]

It could easily be all A36 in that time frame. THere's also no way you can assume composite design here, especially in heavy industrial

 

[ThorenO2]

Thanks @canwesteng, I appreciate your input. I was having trouble seeing the advantage of the building's designer selecting a composite deck type for non-composite construction. It seemed that the composite deck type would be significantly costlier for an addition of this size versus say a 2C Conform deck, which can obtain similar superimposed load values.

 

[canwesteng]

I think the cost of the deck is a rounding error - but also it could have been used as a composite floor deck, with no studs, just to save on bar. Not sure what the deck thickness looks like above.

 

[phamENG]

I agree that it could very easily be A36. But it could also very easily be A36 with an actual F[sub]y[/sub]=55ksi. A992 came out largely as a result of A36 being too strong (it has no maximum strength value) which caused a lot of unpredictable failure modes in seismic events. Strong column weak beam is great...unless your weak beam is almost twice as strong as you expected and your column yields first. Boom.

Obviously you can't just assume that - or at least you shouldn't. If you have a critical area that needs to be stronger than A36 minimums, have a coupon taken and tested.

I'm no expert in composite construction (I've only messed with it a couple of times), but I've talked with engineers who just spec composite deck all the time. Not consciously, but it's in their standard notes that they never read/revise. So that's a possibility. It's also very possible that it's all composite. The problem is that you just don't know, and you can't rely on something you know nothing about for a critical application. If it's critical to know, you can scan the slab and see if anything shows up. I've never done that, so I'm not sure how reliable the results would be.

 

[Spanky7]

For your time period, you most likely have A36 steel. A992 did not come out until around 2000 and did not become the preferred material specification for W-Shapes until around the 2004 to 2005 timeframe. I agree with phamENG, if you need strengths greater than the minimums of A36, then have some coupon samples taken and tested.

In my experience, it is extremely rare to see composite floor construction in Industrial Buildings. So unless you have original design documents showing composite floor construction, it would be highly advisable to assume a non-composite floor.

 

[ThorenO2]

Thanks @Spanky7, I found other posts here resembling your timeline - some saying earlier than '04/'05 and all stemming from Northridge (94) as @PhamENG indicated. I analyzed the typical bay for 250 psf LL as a benchmark and determined the typical beam fails with a non-composite A36 design and checks out at 50 ksi.

I had to figure it out for myself why composite construction should be ruled out for Industrial Buildings, so I analyzed composite sections and found they'd be smaller than the brute W24x55 if designed to capacity. But no sense in picking a smaller beam with less connection capacity and all the work of headed studs, so that's settled.

Since an A36 floor would only be good for 200 psf, which does not match up with conventional heavy industrial loading, it is my opinion there is a high probability that 50 ksi steel was used to get the 250 psf LL capacity, but I know that that isn't a guarantee and more work needs to be done.

 

[canwesteng]

What is the floor being used for? It could be 100 psf for industrial if just maintenance. If it has forklift access probably should still be more than 200 psf.

 

[ThorenO2]

At least 250 would be warranted. Forklifts all day long and heavy manufacturing, punch presses and the like. I was able to find a Google time-lapse, separate from Google Earth which definitively shows the roof appear in 1999. It showed up on a relatively pixelated map as an unmistakeable 900' x 125' black strip. Still not conclusive as to steel grade, but with A992 not exclusively in use at the time, it may be A572 50ksi. It is unlikely the building was designed for only 200 psf, since there is giant concrete pylons in places that supported the original manufacturing operation.