PEMB Column to Beam Stiffness Ratio

[RFreund]

Are there any rules of thumb or typical ranges that PEMB designer's use for the column to beam stiffness ratios (typical ?

Basically we were asked to do some preliminary foundation design for a PEMB however the thrust at the base of the column is based on the moment in the column which is based on the relative column beam stiffness. The Arch and Owner understand that we must make this assumption but they are looking for cost feasibility at this point, I believe.

Another question - we seem to see mostly pinned bases for PEMBs is this common as well?

EIT

http://www.HowToEngineer.com

 

[JedClampett]

This question is best posed to a PEMB designer, but I'll give it a crack anyhow.
As far as pinned bases, that's the normal design. It's just too hard to develop moment in the base connection.
The relative stiffness is tricky. As you probably know, it's very common for PEMB manufacturers to taper their columns and beams. At the top, the stiffnesses are identical. I would use a ratio of 2 (beam over column), but that's subject to debate.

 

[CTW]

Bases are typically pinned. Nucor Building System's Product and Engineering Manual is online. It may offer some insight into the column-beam stiffness ratio. Here's the section on foundation reactions.

http://www.nucorbuildingsystems.com/nbsproductandengineeringmanual/09_00.pdf

 

[jimstructures]

Pinned bases are certainly the norm, 95% to 98% with a few fixed base columns for wind post bracing, crane columns and endwall posts. Remember that the PEMB designer desn't have to develope the moment in the base connection they merely tell the foundation designer what the moment reaction is and the foundation designer has to come up with the fixed foundation. At the top of the column/end of the roof beam the moment is the same, not the relative stiffness. the depth of the roof beam may be much greater than the depth of the column. A better correlation might be length of the column compared to some portion of the length of the span of the roof beam. A longer column can develope a larger moment at the knee with a smaller reaction at the foundation.

Hope this helps

Jim

 

[RFreund]

Thanks for the replies. And sorry for the OP, I need to proof read better.


Jed - I was thinking about the same.

CTW - I looked into the Nucor manual (thanks for this by the way) and at first I thought that the Y direction was perpendicular to the wall and thus the thrust load or kick due to the vertical roof load (which is what I am looking for) However when I look at 9.1 (

http://www.nucorbuildingsystems.com/nbsproductandengineeringmanual/09_01.pdf

)of their manual it looks like X is actually the thrust direction. Do you know which is correct?

Jim - Basically I am trying to gauge the moment at the top (fixed) end of the column so that I can approximate the 'outward' thrust (shear) at the pinned base. Is there a % of total moment then that would typically go to the column?

EIT

http://www.HowToEngineer.com

 

[jimstructures]

Any builder/dealer for a pre-engineered metal building mfg. should be more than willing to provide reactions for typical buildings based on your local codes (We certainly would.) This would give you gravity, uplift and horizontal thrust reactions at the base of columns and a typical anchor bolt detail. What else are you looking for?

Every company has a proprietary scheme for proportioning their frames locked into their software which is going give slightly different results but this should be close enough for estimate work. Of course you are going to have throw them a building every now and then.

I do not know of any published work which starts from the dimensions and gives you the reactions with out runing a frame design program.

Hope this helps.

Jim

 

[RFreund]

Jim - You are correct, that's what I'm looking for. Normally that is what happens - the Arch gives us some preliminary numbers from a prlim done by a PEMB mfg and we use these. However I believe they are going to get multiple bids and want to include a number for the foundation. However I'm not sure why they don't want to ask for a prelim design as a basis...
In regards to getting reactions without running the program - I agree, I need to know if the beams are going to be super huge and thus carry basically a simple span moment with not much moment going to the columns or vice-versa or somewhere between. I'm assuming between and I think Jed's suggestion is a decent place to start.

EIT

http://www.HowToEngineer.com

 

[JedClampett]

I think Butler might help you out and I'm pretty sure that some of their competitors (Varco Pruden, Ahrens, etc) would too, especially if you mention they're in competition with Butler.
I tried to do my own recently and was pretty far off. I thought the PEMB manufacturer's were close to code, but it turned out they were more conservative than I thought.
So if you're estimating, add a decent fudge factor. And make sure you have a plan for anchorage. If you need to include details showing bolt spacing, don't be afraid to to that. Otherwide you're going to get a 4 inch by 4 inch bolt pattern and have to make it work.

 

[fancypants]

I deal with this sort of thing on a frequent basis. Pick something and stick with it. Note exactly what you did and your assumptions in the notes. Also in the notes state that the final foundation/footing size design will be required after you get the "SIGNED AND SEALED" PEMB drawings. Notice I emphasized that or you might find that they change it on you for the build drawings. Note that the "Signed and Sealed" reactions will be required for final design.

If you tell me the bay size and width of the building, I can look on some previous PEMB reactions and get you close. Matter of fact, I just looked at the reactions for a 120 foot wide building ( 3 columns across that is two drive bays) with 25 foot long bays. The kick out reaction total from dead, collateral and live is 7 kips. Actually I looked at about three different sets and 7 kips for a 25 foot bay seem about max.


On another note, 99% of the time the columns with be pinned base. I even specify they should be so in the general notes. However, 99% of the time the only thing the PEMB guys will see is the Architectural drawings. On a few occasions I have gotten bit and they showed moment connected bases. It the building is pretty tall and not too wide say width and height about the same, and the building has cranes, they will show the bases moment connected because it save them a butt load of steel and puts all the problem on you.

Don't forget to add the crane loads to your footings if they are in the building. Also typical loads they will run is dead 3psf, collateral max 5psf unless someone tells them different up front and live will be a reduced 20 psf.

 

[CTW]

The X Reaction is horizontal so X is the thrust. Y is the vertical reaction.

 

[RFreund]

Thanks again guys.

Building is 80' x 160' so frame width is 80 feet. Bay spacing is typically 22' with (1) worst case of (2) adjacent bays at 36 feet. Columns are 20' tall on one side and 32' tall on the other.

Bay spacing of 36' and Ib/Ic = 2, I get about 29 kips, that seems a bit large.

Bay spacing of 22' and Ib/Ic = 2, I get about 17.6 kips.




EIT

http://www.HowToEngineer.com