Prefabricated Metal Frame Analysis

[autokoz]

Renovation of existing frame that was a prefabricated metal frame with taper column and girder frame supporting thin wall box purlins (severly corroded to be removed).

Any suggestions on simple analysis to determine existing frame capacity?

 

[youngstructural]

I assume these are tappered portal frames... Do you have the original drawings? How much information are you able to obtain? Depending on how familiar you are with cold-formed steel and buildings using same for bracing, etc, you may want to read "Bracing Cold-Formed Steel Structures" by Thomas Sputo & Jennifer L. Turner.

Remember: Tappered columns and rafters are meant to match the required capacity at given locations and generally result in economies when material costs outweigh labour. If you consider the bases pinned, and reverse engineer the bending capacity of the sections at the knee, you can work out the lateral load capacity for the three "old fasioned" cases (ie: wind against both walls, wind on one half of roof, and wind on full roof). The UDLs in each case that produce the knee BM (be careful to check mid-span rafter BM as well) will give you your capacity.

You may also wish to include a nominal fixity at the base (perhaps 10% of EI as a maximum). Moment redistribution is also possible, but you did ask for simple, right?

Let me know if you don't have tables of portal frame analysis formulae, I can scan and post if need be.

Good luck,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[dik]

If these are pre-engineered metal buildings, they generally have little residual strength above the design values and safety/load factors.

You have to be cautious about any additional dead loading... I was involved in one study where the roof deck was corroded so badly that the new deck and insulation proposed generated $.5M repairs... the building ended up abandoned.

Most metal building outfits will not review these... even if originally fabricated by them... they sold it, they built it and that's their involvement.

Analysis is very time consuming and plate thickness are incremented by 1/16's of an inch... little slop... and stability issues are also critical due to the long clear spans and changes in moment envelope with varying loads... Also have to check original loadings with current loadings. In Manitoba they have gone from a 1/30 year return period for loads to a 1/50 year with a subsequent increase in loading...

Dik

 

[twinnell]

don't know of a simple analysis. We have analyzed a few of these. We take the shop drawings, if available, and actually imput the members into Risa or Staad to analyze frame. If we don't have drawings, we field measure, with calipers, the member sizes. That gets expensive. Be sure to input the reduced sections due to the corrosion.