Yield Strength of Steel Joists 1

[UnusedHandle]

I am checking an existing building for the addition of new equipment that will be suspended from open web joists. The drawings list a variety of 22H and 24H joists in the different locations. The added equipment will increase the moment in the steel joists beyond the published acceptable value and the customer wishes to suspend between the panel points of the joists in order to align with floor mounted equipment for the assembly line. We can add a new internal chord at the hanger location, but still need to check the joists, so we are analyzing them as trusses.

The issue I have is that the publication I am looking from Vulcraft lists a base yield strength of 50 ksi for the chords of H-series joists. The building was put up in the 70's and the chords are only 2x2 double angles. I find it hard to believe that they have a yield strength of 50 ksi, yet I can't find any publication to support me, or I overlooked it. My preference would be to assume A36 for all of the members. Does anyone have an alternate course of action?​

 

[jayrod12]

Are they vulcraft joists? Are you using their guide from that era? I would lean towards A36 as well based on age and shape however I do know that some stuff was manufactured with 50ksi so it's not totally far fetched. If the joist is in fact a vulcraft, and their book from that era lists 50ksi, then that's what I'd use.

 

[XR250]

There were some 50 ksi stuff back then. Pull a tag off the end of a joist and call Vulcraft. They usually can tell you although it may be too old.

 

[SteelPE]

You can pull a tag, but I doubt Vulcraft will be able to tell you much more. I have had very little success with them in the past.

I highly doubt there is much additional capacity in the joists. The manufacturer is in the business of making money not wasting material. Most of the modern designs I have seen have the members stressed to 98%-100% of their capacity. You could reverse engineer the joists to see if it is 50ksi or 36ksi.

 

[SlideRuleEra]

50 ksi steel was first used in LH-series steel joists in 1962. By the 1970's it probably was available in the H-series, too. See Matthew Stuart's article in Structure Magazine at this link:

Antiquated Structural Systems Series - 9a - Open Web Steel Joists - June 2009

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[XR250]

I've talked to an engineer in the Vulcraft SC plant on a number of occasions. He has helped me identify many joists.

 

[Stenbrook]

Last time I checked with Vulcraft, they only had jobs from a certain time period ( I believe 1980 and newer) and they had removed the older jobs. But most of the time they are very helpful. We usually assume 50 ksi steel as well.

 

[Once20036]

When I've run into a similar situation, I've tried to look at the whole picture. Older SJI specs will sometimes say that the chords are 36ksi or 50ksi.
Based on the measured steel sizes, you should be able to determine allowable compression in the top chord, allowable tension in the bottom chord, an allowable moment, and the uniform load that corresponds with this moment.
Do this for 50 ksi and 36 ksi.
Then, calculation the moment of inertia (with 15% reduction per SJI), allowable deflection, and the uniform load that corresponds with this deflection.
Lastly, check the load tables. I expect you`ll find a good match for the 50ksi design. The 36ksi results may appear a little bizarre - like the allowable load for deflection is greater than the allowable load for strength.

The first time I ran into this situation I went through the above procedure, convinced myself it was 50ksi, and then pulled a coupon for tensile testing to confirm 50ksi. If I recall correctly, this building was from the late 70s.

 

[JedClampett]

IMHO, you shouldn't assume anything beyond what's conservatively reasonable. In other words, unless you have information from the joist manufacturer, use 36 ksi. Joists are inexpensive members made to support exactly the live plus dead load posted and not much more. Why put your license and reputation and possibly lives at risk to save the client a very little money? Put it this way, they saved the money with the joist system when they built the building. The trade off was a loss of flexibility in the future. I know the building has probably changed hands many times and the people who made the decision to use joists are long gone. But oh well, that's the deal.
As you might or might not be able to tell, I'm not a fan of joists. Besides their inherent flimsiness, their customer service is HORRIBLE. How many posts do we have in these forums asking for data on some obsolete or non traceable joists? Vulcraft is one of the better ones and they're not great. Arguments about the way I specify them, about the loads, and about many things they won't do.
When I'm forced to use them, I never specify them by load. I pick a joist one or two sizes larger than I need. If a 18K7 works, I use an 18K10. That way I know the capacity and retain a little control.
Sorry about the soap box. But bad customer service irks me. In my small world, I won't tolerate it.

 

[UnusedHandle]

Thank you everyone who has responded. I am trying to get someone from a satellite office to go to the facility and pull a joist tag. I've considered the option of back-solving, as suggested by several, however this is one of those jobs where it was sold as "those look OK, we'll just assume that they'll hold" and then have no time allotted for making sure we don't pull the roof down. If we can find a tag and have everything verified then, we'll take that route. If not, then I'll burn up the hours we don't have and argue safety and liability afterward.
@JedClampett, I couldn't agree more. Every time I see joists, I know it's not going to go well and I'll have to explain to a salesman that steel isn't adamantium, and we actually have to check to see if it can carry more loading.