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Determining capacity of steel bar joists from 1955

Determining capacity of steel bar joists from 1955

Determining capacity of steel bar joists from 1955

(OP)
I'm working on a project evaluating the capacity of a partially collapsed roof. The steel bar joists are from 1955. I have the dimensions and am looking for some resources to evaluate this. I was thinking of going to the Nucor catalog and using current capacities, assuming the materials and capacities haven't changed too much over the years? Any other suggestions? Thanks

RE: Determining capacity of steel bar joists from 1955

You need to obtain a "STEEL JOIST INSTITUTE 60-YEAR MANUAL" or "75-YEAR MANUAL" or other available. I have a 60-year manual which i have found helpful on numerous occasions. In most cases, there will be a physical steel ID tag on one end of every joist, which, hopefully you would find in the 60 or 75-year manual.

I suggest that there are many differing configurations that have evolved and changed over the years. I would NOT depend on a current catalog other than for an approximate guide.

In many cases, without the tag, i've had to take measurements and calc all necessary physical properties and "do the math..."

RE: Determining capacity of steel bar joists from 1955

The tags, many times, do not provide you with the joist size information (for some unknown reason)...only a project number and a joist mark ID.

You typically have to contact the manufacturer (if they are still in business) and see if they can dredge up their old calculations.

Otherwise, you can estimate the originally intended joist capacity based on span, spacing, original roof dead load, and original code snow load.

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RE: Determining capacity of steel bar joists from 1955

JAE, i certainly respect your experience(s) and i agree that many times you may not even find a tag.

However and fortunately, from my experience, close to 50/50, i have found the tag WITH joist size which has also been found in the manual. Please see attached...

Therefore, do not give up on the quest for a tag and the usefulness of the manual. I've had success on many more than one occasion!

RE: Determining capacity of steel bar joists from 1955

Well it looks like those were all CECO joists - I've never seen Vulcraft do that so it probably does depend wholly on which company produced the joists.

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RE: Determining capacity of steel bar joists from 1955

have done all of the above, probably ending up having to do what JAE says more often than not.

I suppose this is obvious, but might mention it just in case ....
OP: "I was thinking of going to the Nucor catalog and using current capacities, assuming the materials and capacities haven't changed too much over the years? "

Steel grades utilized in 1955 were, generally if not universally, significantly lower than they are for the current joist industry catalogues.
Look also at slideruleera's .com site. He may have catalogues there matching what you have discovered in the field.

What precipitated the partial collapse? Was it a joist? or something else?
Good luck!

RE: Determining capacity of steel bar joists from 1955

I investigated an existing bar joist roof for a client just the other day. I measured up the joist using my handy micrometer. Asked the client when the building was built. Came home, opened up my 75 year joist manual and found the chord angles listed in the size chart at the beginning (an exact match). Went to the table for the corresponding year and bingo, the joist size (16H4). It did help a bit that the 16H4 was the smallest joist available. But getting all the information helped a ton.

I do not recommend going today's joist catalog and obtaining a capacity as they used different materials back then. The 75 year joist manual will help with that (they should have a spec for that year). I highly suggest picking up the resource if you are ever going to do this again as it can tend to be quite valuable. In my instance above, it saved me 4 hours of analysis.

RE: Determining capacity of steel bar joists from 1955

The SJI has the below link for a resource. A guy will give you a joist designation estimate. They are usually conservative (as I've sent them the measurements asked for the size and compared it to the drawings when they became available later). So they cover their tail, and I don't blame them one bit, but it's a great tool.

https://steeljoist.org/joist-investigation-form/

RE: Determining capacity of steel bar joists from 1955

You can't go by chord sizes to determine the joist designation. They often substitute heavier chords based on availability and what's in stock. Plus different manufacturers often have different standard chord sizes for the same joist.

RE: Determining capacity of steel bar joists from 1955

My experience with the SJI investigation link is that they'll tell you it's the smallest size available based on the depth of the joist, even when the chord thicknesses and calculations indicate that its 3 or 4 steps stronger. I don't bother calling them any more unless its an oddly shaped joist and I can't identify the manufacturer.

RE: Determining capacity of steel bar joists from 1955

(OP)
Interesting. I'll keep all this in mind because I may be doing this much more often now. Another thing I noticed is that there is no information for metal decking in these tables. I searched google for metal deck loading tables from 1955 or historic, but didn't find anything useful. In terms of metal decks, is it reasonable to assume that metal deck capacities are 20% lower than modern capacities based on material advancements?

RE: Determining capacity of steel bar joists from 1955

(OP)
I'm trying to determine how the failure of this metal truss/metal deck system failed. The roofing contractor was repairing the roof and moved all the ballast from the roof to an approx 25'x25' area. The roof was still intact at this point but then a heavy rain storm came through. At this point the occupants noticed noises and vacated. Shortly after the trusses failed. Based on my calculations the trusses were on the borderline of failure. Allowable load from 1955 tables is 247 with calculated 237 PLF load assuming uniformly placed ballast (it was weighed after failure) and 3" of water. The metal deck was heavily corroded and puddle welds between the deck and trusses compromised. My question is how the connection between the truss and deck affects the capacity of the trusses and or deck for vertical loading. I'm trying to think through all the things that could have caused this. The trusses were 25' in length, so the entire truss was loading along it length (reportedly).

RE: Determining capacity of steel bar joists from 1955

Look for a post from "Slide rule era" in other questions for his website. Great historic info.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

RE: Determining capacity of steel bar joists from 1955

The truss may have relied on the deck to provide lateral support. Is it possible the top chord buckled sideways?

BA

RE: Determining capacity of steel bar joists from 1955

(OP)
That was my initial thought. There was diagonal bracing provided between the trusses at the third points though. I do think that the deck was providing additional lateral support but not sure if I can quantify it.

RE: Determining capacity of steel bar joists from 1955

Diagonal bracing was always for the bottom chord bracing the way I've understood it. The top chord is considered braced by the decking. If the decking were to have been compromised then it is possible both the top and bottom chords were considered unbraced which could lead to collapse.

RE: Determining capacity of steel bar joists from 1955

If these were truly bar joists, then top and bottom chords consisted of two bars each. The radius of gyration of a bar is pretty low (one quarter of the diameter), so bracing at third points is likely not sufficient.

BA

RE: Determining capacity of steel bar joists from 1955

In 1955, the allowable tensile stress for bar joists was 18 KSI. This was unchanged since bar joists were introduced in the 1920's. IMHO, it was kept low out of concern for the numerous welded joints, not quality of the steel used.

Available steel had long been good enough to justify a somewhat higher value; for example AISC had raised allowable tensile stress for structural steel from 18 KSI to 20 KSI in 1936. During World War II, AISC increased it to 24 KSI, then returned to 20 KSI in 1946.

You may find that an original shop welded connection failed somewhere in one of the heavily loaded (but not overloaded) joists. Lack of lateral support from the corroded steel deck could have contributed to the weld failure.

Bar joist allowable tensile stress increased from 18 KSI to 20 KSI in 1959.
In 1961, it was increased again, from 20 KSI to 22 KSI.
Steel didn't just suddenly improve, but both bar joist fabrication methods and weld reliability did.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Determining capacity of steel bar joists from 1955

(OP)
Interesting. I didn't think about the possibility of poorly constructed welds failing. I will see if I can get access to the scrap pieces. Thanks for the info everyone, very helpful

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