Steel Open Web Joists (cutting existing)

[woodman1967]

I am working with a mechanical crew on a renovation of a steel and masonry building. The mechanical crew want to bring a new airhandler unit into the mechanical room. The issue is the size of the unit, it is too large to fit through any existing openings. Their solution is to open up the roof, originally that was fine as the original drawings (no as-builts available) show the open web steel joists at 10' OC and the unit is 7' wide. Well, as often happens the actual did not match the drawings. The joists are 43" OC. Now they want to support the roof temporarily and then cut an existing joist and lower the unit in. Then add a bolting detail to reconnect the joist. There presently two joists with a bolted connection at midspan, but this was a retrofit and the joists were designed from the outset to be built this way. Is this doable or should it be discouraged? There are no specs on the joists or even a manufacturer on record.

This is outside of what I am comfortable as I deal mainly with wood and some light steel work.

 

[Ltwine]

"There presently two joists with a bolted connection at midspan"

Not quite understand. Are those the joists 43" apart, and to be temporarily removed? How the joists were supported? Need more details.

 

[SteelPE]

So you basically want to cut out a section on an existing joists (say 10’), install new equipment and then splice the joist back together?

It may be doable but that seems like a lot of work to be done on the joists that are typically made up of small/thin members (cutting, welding and bolting). I wouldn’t think this would be economical either. What about completely removing the joists and then installing new joists once the mechanical equipment is in?

 

[woodman1967]

Ltwine: I probably wasn't too clear about the two retrofit joists. These joists were installed in a different area and were placed directly beside an existing joist. This is to support a unit above. The only reason I included this was to mention it was done, and SteelPE the top and bottom chords of the retrofitted joists are much more substantial than the existing joists.

SteelPE: You are right that typically it would make more sense to replace the entire joist, but they want to do it from the inside and there are several ducts and supports in the way. (in the area they want the unit).

 

[SteelPE]

So it appears that some new spliced joists were installed for RTU’s in a different area of the building. Are you responsible for these as well or was this done prior to your project?

So it appears spliced joists are currently out as an option due to interior problems with duct work and piping?

I have no problems with modifying an existing joist. However, when the modification becomes too extensive I usually will install new spliced joists in lieu of modification. I just don’t like the way it is handled in the field sometimes. If you get a good iron worker quality may not be an issue.

 

[Ron]

It would probably be easier and would certainly require less skill to support the roof and knock out a masonry wall section. That could then be put back without a great deal of difficulty. It would save effort, time, money and you wouldn't need a crane.

 

[Ltwine]

As manuf like to throw in odd shape/size for joist members, you might encounter some technical problems when reconnecting it. I think Ron's recommendation is worth considering.

 

[paddingtongreen]

Design replacement trusses that can be assembled in place, sections not enclosing ductwork can be completed in the shop, then all splices are prepared in the shop. It's a lot easier than cutting the existing joist so carefully that the material can be re-connected with full strength splices.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[UcfSE]

Have you considered removing the joists needed and just replacing them with new ones, no splicing worries? They may even be designed and placed, with any additional framing needed, so that the roof doesn't need to be shored. Of course, I would go with Ron's suggestion first.

 

[dik]

If you are changing the roof 'layout', you could be causing snow accumulation and this load can be greater than the RTU.

Also, you may be affecting the 'cut' joist near mid span. What was once a tension member can now become a compression one.

My $.02... Dik

 

[efsinc]

what is the span of your joists?
are there any new loads to be imposed upon the exiting joists or do they remain the same when the work is finished?

regarding the existing bolted splice you say... "but this was a retrofit and the joists were designed from the outset to be built this way" and i am not sure i understand what you are saying. I have designed/built many roof systems that had a center span bolt splice simply due to the length of the joist, ie. 100'+. this was the only practical way to have them shipped to the site.

The cutting and resplicing of an existing steel joist is not difficult. I would recommend a welded splice. The welds should be designed and must be performed carefully so as to avoid undercutting, thus damaging the joist. If the chord (for example) is cut and then respliced with equal or better section steel splice and proper welds, you are good. do it very conservatively. the cost difference between a 3sq.in net area steel splice and a 4sq.in net area steel splice is v v small.

what is more difficult is designing a retrofit of an existing joist to support new and greater loads, and especially if there is a dynamic loading condition such as an air handler rooftop unit subject to seismic loads.

 

[BAretired]

If it is possible to bring the unit through the masonry wall, that is probably the easiest solution.

BA

 

[paddingtongreen]

Without knowing more about the joists and the roofing, it is flailing in the dark to give advice. For example, some of the older, smaller joists used "top hat" shapes for the chords, I don't know how you bolt them, or weld a butt joint "in-situ".

I agree with those respondents who suggested going in through the wall.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[woodman1967]

Thanks again to all,

I felt I should just update all of you on what was decided in this situation.

I was able to discourage them from repairing the cut truss. Instead a full length beam will be put in place to replace the cut truss.