Existing open web steel joist reinforcement

[Lungshen]

Can anyone recommend a good computer program/design aid for checking existing steel joist capacity for new concentrated load and design chord/web reinforcement for moment and shear?

I have seen people setup their own spread sheet but just wondering if there is such program out there.

Thanks in advance

 

[TehMightyEngineer]

RISA is my only recommendation.

 

[blybrook PE]

RISA does not adequately model open web steel joists as they are all proprietary and a coverall spreadsheet does not exist that I'm aware of. Having a copy of the joist manual (75 year history) is great when doing renovation work to find the loading properties that the joist had to meet, but that's about as good as it gets for a reference manual.

There is not a good reference manual that I'm aware of for the old manufacturer files that gave you the load tables and design properties for each joist. If I tell you that it's a 12k4 from 1974, there are probably 6 different styles of webbing or manufacture to create that joist type, but each will have a different design property that would need to be researched.

Edit for clarification - Risa will work fine when you have true field as-builts of the joist in question. If you have shop as-builts or as-built drawings, it isn't worth a hoot as there won't be enough information on the drawings to adequately model the member. We all know how easily getting to the field to get these measurements can be.

 

[Lungshen]

thanks for the reply, guys. After some research I think I have found my answers

 

[TehMightyEngineer]

@blybrook: Yeah, I've modeled many an open web joist in STAAD (or RISA) and you are entirely right. Without field dimensions it's not worth the time. Plus, the field dimensions have to include offsets, every member size, bracing, support connections, etc. I've easily spent an entire day field dimensioning open web joists and that was with a ceiling level catwalk running parallel under the joist, on a man-lift or whatever it could be really time consuming.

@Lungshen: However, you can usually drag up the steel joist manufacturer's original design by purchasing old or combined historic design manuals from the various manufacturer. Of course, this requires knowing what the original manufacturer was and what the joist designation is. Not always available but worth looking for.

Keep in mind that many loads have changed over the years, especially snow loads. I often run into 60 year old trusses that were designed for 40 PSF snow before the ASCE or BOCA really did any work on minimum snow loading. Same with wind and seismic.

 

[McEngr]

James Fischer has a good article in the MSC magazine (it might be in the STRUCTURE magazine). Also, vulcraft has good info which is common industry practice for reinforcing bar joists.

I somewhat disagree with blybrook because RISA is just a tool (like a calculator). If you know how to use the tool and the limitations to which you are applying it, it is possible to make an analysis or gather information/knowledge of the joist's behavior.

In my opinion, vulcraft and canam design their joists to the gnats rear and don't consider P+M for the top chord. They only design the top chord for the couple that one achieves from a Mmax. I'm not sure how vulcraft and other manufacturers get away with this pretty serious oversight.

Also, if you're curious, the local buckling will almost always control for the reinforcing element and not shear flow. I'm sure others see this as obvious, but I've done many challenging joist reinforcement issues with big HVAC units on cold-storage warehouses and offices. It's not an easy assignment.

 

[ADB]

McEngr: what is typically your findings when you are installing these rooftop units on buildings? Do you often find that the existing situation is ok to support the load or do you have to reinforce the structure somehow? Just curious.

 

[McEngr]

Almost always, there is an A36 rod that is reinforced for only a portion of the top chord unless the roof top unit is very, very large, then it may have to be post-tensioned somehow.

 

[Mark Leyner]

It used to be common practice at Vulcraft to design joists for DL/LL. i.e. - EOR specifies uniformly distributed dead and live loads for a roof, Vulcraft has engineers at their plant that run software to arrive at optimized joists for that loading and the other relevant conditions. In addition, most Vulcraft joists were designed with a 500 lb concentrated load allowance at each panel point. Most girders were designed for an additional 1000 lb concentrated load at each panel point.

It is incredibly uneconomical to specify "standard" joists. At least from Vulcraft's perspective. It isn't rational to have joists and/or girders in stock, so all jobs are custom built. Accordingly, it is economical to design the joists and girders for actual load conditions rather than build the SJI "standard" joists. For the most part, design/specification of joists is highly customizable.

With all that said, your joists may have been designed for the additional loads at panel points and they may not have been. There are likely tags attached to the joists with identifying information. My guess is that if you can find a tag and that information, the joist manufacturer may be able to tell you what sort of additional capacity (if any) you have.

Good luck!

 

[MA_PE]

A key factor in looking at existing open web joists is checking the as-built construction so that the panel point and action lines truly intersect where the "idealized" configuration shows them to.

Eccentricties due to the as-built condition may affect the true ultimate strength as opposed to the design strength.

Many times these things never see thier ultimate loads so no problem devleops but if loads are increased these flaws end up controlling the actual test strength.

I have performed in-situ full scale load tests prior to structural modifications to the building in order to 1) demonstrate that the existing joists do in fact have their rated design capacity and 2) validate our RISA model of the truss (which included measuerd offsets).

 

[McEngr]

Mark - I've encountered that those additional loads at the panel points are not concurrently applied. They are only applied like an influence line for a bridge, but with increments of the panel point distance. Also, the reporting in the vulcraft calculations does not include these panel point concentrated loads. Often there will be a stress ratio of 1.01 with no panel point load in the moment calculation. Obviously, it's not hard to find the axial load from a moment, and I know for a fact that those concentrated loads are not considered for the chord calculations.

 

[Mark Leyner]

Mark - I've encountered that those additional loads at the panel points are not concurrently applied. They are only applied like an influence line for a bridge, but with increments of the panel point distance. Also, the reporting in the vulcraft calculations does not include these panel point concentrated loads. Often there will be a stress ratio of 1.01 with no panel point load in the moment calculation. Obviously, it's not hard to find the axial load from a moment, and I know for a fact that those concentrated loads are not considered for the chord calculations.

Thanks for your response. Good to know!