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Re: Are Roof Top Units considered Dead or Live Loads?[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Are Roof Top Units considered Dead or Live Loads?
- From: ad026(--nospam--at)hwcn.org (Paul Ransom)
- Date: Thu, 6 Jan 2005 11:29:24 -0500
> From: "Daniel Boltz" <dboltz(--nospam--at)1st.net> > I've had different answers from various Senior Engineers. Some say that > since the RTU weight does not vary in magnitude and location, it is a DEAD > LOAD. Others say that if the RTU is changed or moved in the future, it's > weight and location would be considered a LIVE LOAD. This has been discussed on the list in the past, as well. RTUs are a little schizophrenic, if that's an appropriate term. They have two natures: 1) they are "static" in that they don't move around without direct intent by the owners. The operating loads and locations, at the TIME OF INSTALLATION, are 100% known or knowable and this does not change. The RTU is dead load. 2) at DESIGN TIME we rarely have good numbers for the actual weights or operating effects of the final units (e.g. Christopher mentioned dynamic issues). Additionally, they can be removed or relocated with some structural detriment. So, for design purposes, we are inclined to think of them as live loads. The real issue comes down to how we, as designers, handle the effects of these RTUs, not whether they should be classified as dead or live loads. The building codes typically place RTUs in the category of Dead loads which, in my opinion, they are. "Dead" loads are treated in a different manner than "Live" loads for the purpose of REGULATORY compliance and for rational DESIGN CONVENIENCE. Dead loads are ALWAYS in a fixed place (e.g. they don't move around on the roof when the wind blows, they don't mulitply like mechanical rabbits, they don't over-eat at Christmas, etc.). The drawings submitted for permit show where that RTU will be - end of story. Relocated dead loads are REVISIONS, not live loads. What are your design and contractual objectives? If you are concerned about the possibility that a different unit will be installed and you want to cover yourself and avoid working on design changes, assume an altered load or different load factors (same net effect). Do not be concerned about whether you use 1.2 or 1.6 because you want to fit it into a nicely defined category - heck, use 2 and forget about it! It's STILL a DEAD load even if you treat it like a live load to be conservative in design. >From experience, we know that things change. Now, we start to see the opposite nature of RTUs. Maybe they aren't installed where we designed the structure to support them. Maybe, they are heavier or lighter than we anticipated, in a detrimental way. Try to cover reasonable probabilities: Heavier - design the support for more load than expected (careful not to be extreme or you can be black-mailed for that as well). Anything else is a redesign issue. Lighter or not installed - uplift may be a problem. Unless your customer advises you that these MAY NOT be installed until a future date, this is a redesign issue. Location - allow a tolerance of installed location based on the plan that you are given. Anything else is a redesign issue (OWSJ supplier will quote/design with loads/locations EXACTLY as defined in the drawings/specs). Future Renovations - A redesign issue. Unless your mandate is to produce a design that would permit defined future conditions, you are done. RTUs are a gold mine for extras and future work. If the RTU specs change between design and installation, it is NOT your fault and NOT your nickel to review and reinforce for the change. The owner should have given you appropriate specifications up-front if they wanted to avoid delays or additional expenses. Their mistake = their cost. -- R. Paul Ransom, P. Eng. Civil/Structural/Project/International Burlington, Ontario, Canada <mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html> ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
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