Quick Survey -- Beam dimension for RTU support

[JKStruct]

I get this RFI on just about every project:

"Please provide the dimension off line 'x' for the W12x19..." supporting the RTU curb. Fabricators get insolent because they can't "finalize" shop drawings without these dimensions. We'll get the cut sheet from the MEP for a TRANE unit with dimensions and weights of the unit, and we provide support. Then we call out the weight on plan and show a beam adequate for support. We never dimension it. I guess the argument is that too many times the contractor will say "I can save $4000 by buying the unit from Mom n' Pop RTU," the mech. eng. gives the ok, but the unit is 8 inches narrower. No one bothers to tell us, so we have a beam in the 'wrong' place, and it's our fault. So, we tend to punt when this question comes up. We tell the GC to coordinate with his fabricator and provide the dimension based on the unit he buys. Is this fairly typical in the industy? Does anyone actually provide that dimension? And if not, why does every contractor ask this question?

As always... I appreciate your input.

 

[271828]

That's about what I've done. Perhaps put dimensions on there and a note to coordinate with the unit. That way, if they don't want to be diligent, it's at least technically their fault.

 

[prsconsultant]

Similar experience several times. In a recent project we had 6 AHU's and all six units ended up different than the original. GC scaled the dimensions of the structural drawings (none were shown but we had a number notes requiring GC to coordinate with mechanical. Shop drawings were approved as it had shown dimensions. The cost to fix this ran up to about 75K. GC is still pointing fingers and trying to get some money from the owner.

 

[DaveAtkins]

I agree with leaving the determination of the dimensions in the hands of the GC. My experience is that good GC's know that this is something required of them.

DaveAtkins

 

[minorchord2000]

I totally agree with Dave. Structural support beams for mechanical equipment whether they are AHUs or chillers or whatever are notoriously difficult to dimension as to their exact location. Most of the time what was specifed or shown is not what is ultimately purchased, unless they are packaged as long lead items. It is sufficient in my view to size the beam and its connection, but its location is the responsibility of the GC. Clearly it is also his responsibility to coordinate such minutia with his subcontractor and/or suppliers. The front end of the specs probably say this somewhere.

 

[MarcbSE]

I am in complete agreement with the previous posts. I size the beam and connections, but have a general note that requires the fabricator to coordinate the location with the mechanical contractor. When the shop drawings come back with big clouds asking for the dimension it gets marked up with "Dimension to be provided by mechanical contractor".

 

[PMR06]

I agree with previous posts, but we still add a little bit of information. We dimension a rough RTU location, call out a rounded off RTU weight and add a note for mechanical to coordinate actual locations. We put the rough dimension and weight so that the RTU is about where we want. That way if it ends up on the other side of the building and double the weight, we can say NO. The rough information is also helpful for the bar joist designer (if applicable).

 

[MarcbSE]

I should have been more complete in my answer - I DO always show the approximate unit weight on the plans and will give an approximate location, but require all information to be coordinated with the mechanical contractor prior to fabrication.

 

[lkjh345]

Well, I'm not going to stop this unanimity. I also show the beam(s) required to support the unit, and require the contractor to coordinate the exact location(s) based on what brand of RTU he ultimately chooses.

For what its worth, when sizing the RTU support beams, I generrally size them for a unit 50% (for larger RTU's) to 100% ( for small RTU's) heavier than what the mechanical engineer tells me they are going to weight.

I got tired of getting a call from the mechanical engineer, after the steel is all erected of course, to the effect of 'We orginally thought RTU-1 was going to weigh 5000 lbs, but we have had to modify our system and now RTU-1 weighs 10,000 lbs. Is this OK? '

 

[whyun]

I agree with the above that Structural Engineer should not provide the dimensions, though they may be fully capable, or in fact, the most competent discipline to figure out the correct dimensions.

More generally, I only indicate "structurally significant" dimensions on the structural drawings. Dimensions that actually matter for the structural design (column grids and so on). Floating dimensions affecting architectural, mechanical units and so on MUST be "fixed" on the drawings by SOMEONE (definitely not structural) and my opinion is that it should be the discipline that warrants the piece to be located. It is unacceptable and unfair to the contractor to figure this out regardless whether they are competent enough to figure out themselves.