Unistrut Seismic Bracing Systems

[awa5114]

I am going through an example in the Unistrut Seismic Bracing Systems design guide (

http://www.unistrut.us/E-document/Unistrut-Seismic-Bracing-Systems/#?page=54).

In an effort to uncover the deeper design principles behind this, I have been scouring the net for other similar design examples. It turns out many manufacturers, including PowerStrut (

http://www.power-strut.com/DB/PDF1/PS0901_Seismic_08.pdf)

and Cooper Industries have extremely similar examples in their documentation, only the pieces themselves are slightly different. In several cases the same professional engineer has stamped these calculations.

My question is, beyond brand names like those mentioned above, does there exist a generic design procedure for trapeze and pipe hangers with their seismic bracing that is agnostic to the brand name? I am quite frustrated by the state of the design examples in these manufacturers documentations as they are teeming with mistakes and lack of explanation.

 

[Teguci]

Power Strut = Unistrut (same company and they've even started stamping both part numbers on their fittings).

Not sure what you are looking for regarding generic design procedures

 

[awa5114]

Please check the following link, it contains the example Im referreing to (

http://www.unistrut.us/E-document/Unistrut-Seismic-Bracing-Systems/#?page=54).

This catalog contains a trapeze hanger example, and so do the other manufacturers. My question is, is there a more in depth technical/design document which elaborates on the design of these hanger type structures, the underlying assumptions, and high quality detailed examples on how to design these things, irrespective of which manufacturer I'm using.

 

[wannabeSE]

If you have any specific questions, I may be able to help. The closest thing that I have seen for free is Seismic_Design_and_Retrofit_of_Piping_Systems.pdf. But, I don't think this is what you want. Also, FEMA has some documents, but they more or less present the problem. I haven't seen any include techniques for analysis or design of distribution systems.

The one book I mean to get but haven't might be helpful: ASHRAE's Practical Guide to Seismic Restraint

You might also be interested in SMACNA's Seismic Restraint Manual Guidelines for Mechanical Systems It has been a while since I looked at older editions of this book. But, I don't remember being enlightened after reading it.

If you are doing fire sprinklers, you need to look in NFPA 13 because they have some additional requirements/ methods. The brace design is based on the mass in the zone of influence, so the weight of unbraced branch lines is included in the mass of braced mains.

If you don't mind manufactures. OSHPD has a couple pre approved sets of details with cook books similar to Unistrut but more current. They can be downloaded from their web site,

http://www.oshpd.ca.gov/fdd/Pre-Approval/preapprovalMfgCert-dtaTable.html.

Mason, TOLCO and ERICO are on the list; just click on the link (the others are for anchoring other none structural equipment, everything from monitor mounts to steam sterilizers - all kinds of hospital equipment). The TOLCO and ERICO preapproval is only for fire sprinklers.

 

[Teguci]

is there a more in depth technical/design document which elaborates on the design of [strut]

No, not really. "Slotted Channel Framing" (Strut) has it's own specification 054300 and even has an organization, MFMA. You can glean some information from those documents (simple web search), but not really what you are looking for. The AISI design guide would probably be the best foundation to start from when looking for real structural properties for the design of strut. However, the capacity of the strut nuts in shear and tension is based on testing that is only referred to in the catalogs.

As for manufacturer, you should take the time to review each manufacturer's catalog. For the most part, the moment and axial capacities are identical (+/- 2%) and I treat them as a commodity (except for Hilti because they want to be different).

One important note. A huge advantage for using strut over structural steel is the flexibility in making connections anywhere along the slot. This is also a disadvantage to the designer who thinks his system will be installed to the exact dimensions shown on the drawings. Do yourself a favor and build in tolerances for your calculations. Strut is cheap, but labor is not.