Pre-Engineered Metal Building - general considerations for openings 2

[ThorenO]

Hello, thank you for any feedback in advance.

Existing PEMB Description
360' wide (6-span frames) x 300' long (12) bays
Eave at 23', Ridge @ 33'
1" dia. X-bracing with hillside anchors. Bracing located at (4) of the (12) bays on each end wall. Probable that (1) or more were removed.
8" "Z-shaped" girts at 5' on outside face of frames.
8" "Z-shaped' purlins at 5' at the roof with wind-bracing.
C8 eave strut with 8" strut-purlin at end walls.

Pretty standard.

What's been proposed is to remove ALL the X-bracing on one side to permit 14' wide x 16' high overhead doors in the center of the 25' bays.

I'd like to get some ideas and general considerations on restoring the building's lateral system and creating the openings.

 

[KootK]

@Archie: drilled in AB would definitely have been preferable, both aesthetically and from a durability perspective. I think it was a 250 wall with verts at 300. I wasn't able to make drilled in work with edge distances and such. Of course, I was very green at the time and even more paranoid than I am now.

@AJH: I should clarify that I wan't proposing a three sided system. Rather, I was warning that, if the diaphragm were rigid and the moment frames not sufficiently stiff, the building would approach three sided behaviour to the chagrin of the designer. The roofs of these buildings are generally rod braced and therefore treated as flexible diaphragms, right?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[ThorenO]

Thanks for the great input. For a 25' bay, a moment frame / wind bent just above the door @ 16' with X-bracing above is where I'm at now. The squatter the better. Problem is I've got 15 kips at a 23' eave, so the deflection is very high even with large sections. Can there be a consideration for installing K-braces on each side of the opening? This brings the deflection back to where it was, but I'm wondering if that would be a more expensive / complicated option.

 

[KootK]

The K-bracing would require opening up more wall and, probably, dealing with foundations not suited for use in the lateral system. The later, in particular, could get expensive.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[ThorenO]

KootK - thanks again. As you suggested, the K-brace reactions are way too extreme to deal with. So I should go with (4) massive frames. What sort of shapes would you think are in order for this? If you'll notice in the PDF, the Table 3.2 seems screwed up. But I'm thinking that the drawing is pretty reasonable and I'm looking at W24 columns.

 

[ajh1]

KootK - Yes, the roofs generally are assumed to be flexible diaphragms. You know the 3-sided philosophy doesn't work, but not everyone would necessarily recognize that if they hadn't worked through the problem in the past.

 

[KootK]

I don't really have a good feel for the shapes required ThorenO. I was imagining a frame with a high and low beam supported by continuous columns and cross braced in between beams. The system shown in your PDF seems a bit different. If things were set up the way that I had imagined, I'd do it something like this:

1) Assume that the columns are fixed at the low beam.
2) Hand pick columns based drift limits and Ix. Give yourself some wiggle room (~15-20%).
3) Run the frame in SAP/RISA/RAM to check the strength of your members.
4) Iterate and optimize for as long as there's low hanging fruit to be picked.

What document is it that you've posted the excerpt from? I'm not familiar with it but it looks interesting. The moment framing in the sketch looks like something proprietary?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[ThorenO]

May have to try a two-tier frame next... I attached a STAAD diagram of a 3300# frame with X-bracing and it's deflecting 1.2" vs 0.8" original. Someone told me this is called a "Mill Bent"

I also added the other schematic of a Full Height Portal frame from a book Metal Building Systems.

The pavilion is for a building I'm glad I'm not dealing with.

 

[KootK]

Yeah, the way that you have it set up with the mill bent, the cross bracing doesn't really help resist the lateral load, it just drags it down to the frame. With the two tiered arrangement, the bracing basically acts like a monstrously stiff beam connected to the columns. At least, that's the hope.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.