Load zones for future mechanical equipment

[conradlovejoy]

I work for an A/E firm and often encounter both sides of this coin. We are retained as both the structural engineer to design shell buildings where no mechanical equipment information is available, as well as tenant remodels of existing buildings for which new mechanical equipment loads are added to the roof. When the shell building engineer adds a load zone for future mechanical equipment, does this allow for future building designers to elect not to retain a structural engineer to analyze the addition of new equipment to the roof? My opinion has always been that when the loading of an existing structure is modified, the municipality has the right to require a structural engineer to take some part in the process, whether that be a letter stating the structure is adequate to support the new demand, or drawings that provide reinforcement.

I have a specific case in which an engineer who previously worked here designed a shell building and added a mechanical load zone, and the architect performing the tenant finish out within the shell structure is asking me (as an engineer who still works for this firm at which the EOR worked when the plans were created) to tell him the maximum allowable weight of any one RTU if they are to be placed in the load zone. I feel like there are so many branches to this that involve both legality and engineering. I don't know if my opinion that any project involving load modification could potentially be kicked back my the governing municipality if a structural engineer isn't retained is correct. I am amicable to be helpful, but am cautious not to assume liability where I don't have enough control. My preference is to tell the Architect that I need to be retained as the "new project" structural engineer, but my concern is that this will be met with push-back as unnecessary since the EOR already provided equipment support details and a mechanical load zone. It feels like a strange argument to make that, although a load zone was provided, I would still need to analyze the proposed locations and magnitudes of new loads in order to properly provide assistance as a professional while assuming liability for the performance of the structure under the new demand.

 

[phamENG]

A properly detailed load zone will indicate the assumed point load and where it can occur (point load being the weight of the RTU). Therefore, the contractor doing the build out should not need a structural engineer to review it. If the loading they are proposing is within the limits of those set by the EOR and within the designated load zone, I think your best bet would be to point them to the appropriate locations to find that information on the drawings used to build the building in the first place. I don't think that would expose your firm to any additional liability. Be careful, though - if the joist manufacturer missed it and the EOR didn't catch it on the shop drawings, you could have some issues. Liability exposure in that case would depend on your firm's contract and what you agreed to do in terms of shop drawing review (many firms state that their review is for general conformance only and they do not accept liability for mistakes that aren't caught).

If they have selected an RTU that is larger than what was originally designed for or they want to put it somewhere else, then it's a new project and you should be paid to re-analyze and reinforce the roof.

 

[conradlovejoy]

The issue is that the EOR designated a zone in the "middle" of the roof as the future mechanical load zone and indicated the joists should be designed to resist an additional 25psf dead load within that area. I have access to the calculations which basically just depict that the selected joist adequately resists the plf load introduced by the specified joist spacing, the listed dead and roof live loads over the entire length of the joist, and the additional dead load along the portion of the joist within the load zone. At the time of the design, no mechanical information was available, so the joists weren't designed for specific RTU loads. Now the TFO arch wants a weight limit for RTUs that are anticipated to be located within that zone so he can provide that information to his mechanical designer.

I'm guessing by your first line that its your opinion this load zone is not properly detailed and/or is too vague. The (somewhat hokey) math can be done to provide an answer to the original question, but my concern is more toward what would typically be required from a permitting standpoint in such a situation, and if my opinion about the possible permitting issues is correct, whether its reasonable to push toward not providing additional information unless properly retained.

 

[EZBuilding]

My problem with this proposed method is that the height of these units can vary significantly. This height could impact snow drift weights and wind overturning loads. Unlikely to all have been taken into account by a blanket "add more weight capacity" statement.

 

[phamENG]

I would say that is not a properly detailed loading zone for RTU's as an RTU doesn't create a uniform distributed load, it produces point loads. Joists are pretty sensitive to point loads, so it's an important distinction in this case.

This also puts you in a potentially tricky situation. The owner already paid your firm once to provide a roof capable of supporting mechanical units, likely with the intent of not having to come back to you at this stage. Unfortunately, the EOR seems to have failed in the delivery. Definitely one to take up with a supervisor/partner/department head (don't know what kind of firm you work for or how big it is), but my guess is checking this will be a "complimentary" service since it should have been done under your previous contract. Of course, I could be totally wrong - your previous contract may have been silent on future mechanical units and the EOR did that just to make sure that the joist could globally carry the added loads knowing that somebody would have to detail localized reinforcement later. Again, one to take up with management while reviewing the original design contract.

EZBuilding brings up another good point. There are two sides to a loading zone: one for the joist manufacturer where you provide the zone and the point load to be applied, the other for the future mechanical designer to limit the dimensions and total weight of the RTU. It's folly to expect a mechanical designer to understand what you've given the joist manufacturer or to expect the joist manufacturer to determine the loading based on what the mech needs. Where I am, snow drift is pretty inconsequential (unless you're talking cooler tower on an acre sized roof) but overturning from wind loads can get pretty severe.

No matter what happens, it sounds like this will absolutely need the attention of a structural engineer to confirm the placement of the RTUs. Best bet is probably to give general guidance, and then let the mech pick it out and design around it. There are too many variables with dimensional variations and resulting loading to nail down ahead of time.

 

[conradlovejoy]

phamENG:

I want to make sure I don't misunderstand your last. Your first statement is that a mechanical zone could be expected to provide enough information to allow the mech designer to complete his job without guidance from a structural engineer. Your next statement is that its folly to expect such results. Did you just mean to expect such results given the current information, or any such mechanical loading zone information (because if so these statements conflict somewhat)? I agree with the sentiment that if a mechanical load zone is to be provided, it should definitely include more pertinent information such as 'maximum point load on any one joist within this zone' and the design should include more than just typical dead and roof live loads (i.e. wind loads and snow drift loads as you EZbuilding mentioned). The zone could be simple enough for a decent mechanical designer to glean the needed information, whereas the design behind it could be a bit more comprehensive as we've discussed. I'm now assuming this is what you were saying, which avoids the conflicting of statements I originally misunderstood.

 

[phamENG]

Sorry. That was a quasi-stream-of-consciousness typing episode.

There are two sides to detailing a mechanical loading zone:

1) For the joist designer, who needs to know the loads being applied to the joist (quantitative and qualitative so they can plug them into the appropriate combinations for their analysis).
2) For the mechanical designer, who needs to know the assumed limits for the equipment you used to determine your loading. You'll need equipment weight, but you also need to give them the aspect ratio (width vs height) you used to calculate wind loading, seismic loading, and snow drift impacts for the joists. If the joists and mechanical unit are being designed and installed as the same project with the same design team, this doesn't necessarily need to be on the drawings (though it's a good idea if you can coordinate with your mechanical guys well enough). For a shell building like you're doing, you have to put it on the drawings to make sure it's well understood what the intent was by somebody unaffiliated with you coming in after the fact.

If you do 1 without doing 2, (which is sort of the situation you're in now), the joists will be designed and fabricated and nobody will know any better. The mechanical designer will pick a unit that weighs about what you assumed, but - oops - is twice as tall and narrower and so the actual wind reactions are significantly higher than you had designed for.

So yes, a properly detailed zone will give everyone the needed information. But what a joist manufacturer and a mechanical designer need from the properly detailed zone are different, and both need to be covered.

Hope that clears it up.

 

[conradlovejoy]

That does. I think I am on the same page now, and this actually changed my opinion of mechanical loading zones. Thanks for the responses.

 

[Agent666]

As someone who in the past has checked a lot of roofs for additional plant loads for a services engineering company, keep in mind the original plant will be replaced over time as it reaches its useful end of life.

The next replacement is always heavier/bigger, usually due to being under spec first time round... Almost 100% of the time this is the reality.

So specify healthy plant loads for any dedicated zones. If the original designers had allowed for this it's a quick check to confirm that the structure is adequate, without it you're kept in a job for a little longer...

The original allowance if 25psf (1.2kPa) sounds way too light. Based on my experience allow at least 5kPa for light plant (AHUs, etc), and 7.5-10kPa if you have heavier plant like chillers or cooling towers. This gives the poor structural engineer in the future options. Otherwise you'll be jumping through hoops and probably need additional strengthening or at the very least in some cases spreading any point loads in creative ways over multiple elements to get it to come close to working with a structure that's been designed on the basis of a low UDL approach with zero consideration of point loads.

If you allow for exact weight of plant sitting at exact locations then you're limiting the future case that's sure to come up in 15-20 years when it due for replacement. The higher load allowances also allow you to suck up some of the things like wind loads on units and so forth if you don't know the size at the time of design, again giving options.

Definately make it clear on drawings what load has been allowed