Anchorage for Roof Mounted AHU 1

[terdburd]

Is there a guideline that addresses the requirements for anchoring large Air Handling Units for windload to flat roofs? ASCE 7-02 has yeilded little specific info that gives good insight to this issue.

Most AHU vendors provide sparse details regarding my main concern; that being a clear load path from the base of the unit, thru the curb to whatever structure is available below.

I work here in Florida, specifically Kennedy Space Center. In 20+ years I don't recall an AHU flying off of any roof due to hurricanes or tornados (and we have had our share lately)

Any leads would be appreciated.

 

[JedClampett]

I think the equipment suppliers do a pretty good job anchoring their equipment. We like to put in a wind and seismic specification and reference that in the equipment specification. If you'd like a structural calculation, require it in your specs. The HVAC guys will find someone who will do it.
As far as the curb and steel attachment, there has been much more professionalism in this area recently. They get loads, analyze their curbs as structural members and do a pretty good job of it. Try Roof Products Inc. (

http://www.rpicurbs.com/)

for a manufacturer who takes this aspect of the work seriously.

 

[Lutfi]

They vendors, in my opinion, do not do a good job in Florida. They were able to get a reprieve fro the Florida Building Code. We are required to design the anchorage and the bolts to the latest wind loads in FBC or ASCE 7. However, the AHU and it frames are not. I think this is a joke. Some of these AHUs are huge in size.

I am not sure if the yare still exempt in Florida or not. I have not done one lately.

Regards,



Lutfi

http://www.cdeco.com

 

[jjeng2]

In my experience the mechanical eng. has been responsible for this.

 

[whyun]

jjeng2, Right on!

Actually, mechanical engineers are "supposed" to cover this information, but often they either overlook or simply avoid providing anchorage information.

I practice in California and seismic anchorage is a big deal. I'm sure there are parts of the world where seismic is not a concern, but in any case, one should provide "nominal" attachment governed by equipment geometry and standard fastener size all around the unit as appropriate.

Overturning probably will not be a concern in a non-seismic area.

 

[sigma1]

I agree with Lutfi.
If you leave it up to the Mechanical/HVAC contractor nothing will be done. You have to check sliding and overturning for both wind and seismic loads. It all depends on the geometry of the unit. Some of these units are double deckers and exteremely heavy. The units usually come with steel channel rails. You can either bolt them directly or can use steel brackets to attach them to your steel or concrete structure.

Note: I have seen units bolted to wood sleepers but the sleepers where not attached to anything??????

Good luck

 

[JedClampett]

Two points:
I've attached a link to an article in the February 21 ENR (

http://enr.ecnext.com/free-scripts/comsite2.pl?page=enr_document&article=nebuar050216-1)

that specifically points out that equipment anchorage in Florida was a problem during the recent spate of hurricanes and the code will have to be modified. So expect more attention to this.
Second, if the HVAC contractors are doing a bad job in designing and installing anchorages you're going to have to be persistent. Out west, it's routine to require seismic calculations on equipment. As a matter of fact, ASCE 7-02 now has very extensive requirements in Section 9.6.3 regarding seismic design of mechanical and electrical equipment.
My point is if you put a requirement for calculations and wind anchorage in the contract and keep returning the submittals until you see those calculations, the contractors and suppliers will adjust. They have in high seismic regions. Some education of not only the contractors, but mechanical engineers, might be required.

 

[steve1]

Somewhere along the line the structural engineer has to review the capacity of the roof to not only carry the dead load of the equipment, but also additional forces due to wind, seismic, snow buildup (if applicable), and mechanical forces such as operating and thermal loads.

Often I do the above and also engineer some sort of transfer framework to get the loads to places of adequate capacity.

Typically equipment comes with mounting holes. My understanding is that the vendor is only responsible for an adequate load path to the mounting holes. The design of the bolts is a contractural issue. Sometimes they come with the equipment and sometimes they don't. For my own satisfaction I always check the bolts myself.

 

[whyun]

Steve1,

I agree that structural engineers are the most qualified professionals to check the connections of the equipment subject to combinations of various load cases.

Often the cost of design is ignored by the prime design professional (architect in most cases). It is "assumed" that the mechanical engineer who specify the equipment shall provide the connection detail because they are more familiar with the various units they specify but they are not trained to perform such calculations.

In my opinion, mechanical engineer shall be at a minimum, be familiar with standard connections that apply to specific job conditions, draw the connections, and leave the checking to the structural engineer and pay them a nominal design fee. Often, mechanical engineer are not even aware of "standard connections".

It should be the duty of the design professionals to let the owner be aware of such required design items and associated design cost up front. Also agreement shall be made by the design team as to who will actually perform the work and paid accordingly.

Poorly designed anchorage can result in significant repair costs that far exceed the design cost, let alone legal fees that will determine who pays for the damage repair...

 

[steve1]

I agree with you whyun regarding the fact that costs associated with the design of equipment anchorage should be discussed in the proposal phase and agreeded upon prior to the signing of a structural engineering contract. To often these costs are not considered or considered to be minimal by project managers.

Most of my experience is in the industrial sector where we are a bit more atuned to mechanical installations. I think the problem is with commercial and high density housing projects where there is more emphasis on cutting costs to a bare minimum. Equipment anchorage can easily be overlooked as a design item in that type of environment.

 

[terdburd]

Thanks to all for the inputs. At present, I have had little luck in finding a generally accepted document which specifically addresses this issue.
Apparantly, the practice of merely resting a curb on a flat roof, with little more than a token screw or two, has been around since Noah built the Ark.
This effort I'm involved in is a replacement of an existing make-up air AHU (roof mounted) where the mechanical contractor will re-use the existing roof curb. Records cannot be found that detail the original curb installation, so I am left with performing an inspection of the curb when the AHU is removed and making a real time decision to apply anchorage to the curb if none are apparent.
Will post when this happens and share the experience.
Thanks to all.
terdburd

 

[whyun]

Design lateral wind load on a maximum projected area, I can't imagine, would be large enough to cause overturning forces. The fasteners (lag screws to wood structure, other types for steel or concrete) need only designed for carrying shear forces.

Absolute minimum number of fasteners for a rectangular shaped AHU is 4. One at each corner. If the unit foot print is large, It is not uncommon to see fasteners spaced at approximately 24" o.c.

In California, governing factor is seismic forces which typically cause the unit to overturn. Fasteners are designed for carrying shear, tension combination. In cases of large overturning, sometimes the carrying structural members are checked for the additional downward force.

In your case, if the existing unit is removed and replaced on top of existing curb which is to remain, I would recommend inspecting how the curb is fastened to the structure. If appears inadequate, provide additional fasteners.

Hope this helps.

 

[terdburd]

Florida Building Code Section 1611.1.6 was the guideline I was looking for. The statement in FBC is very generalized and relies upon ASCE 7-20 for the methodology needed to comply with code.

Thanks to all for their inputs, they were illuminating and helpful.