Reducing lateral/shear forces on a two wall structure

[fpr4]

I am having a problem with a simple structure that was built to shelter a cluster of mailboxes. After seeing it built and in place, it's obvious that the culprit is lateral forces causing it to rack from side to side. Steel angles were installed at the base of the columns to prevent the "racking" but the client does not like this solution. We can't put a shear wall on the back of the building because the mailbox cluster has to be loaded from the rear.

Is there any other solution? Here are some images of the building and the initial "solution."

http://farm5.static.flickr.com/4108/5059642475_59bf783f7d.jpg

http://farm5.static.flickr.com/4089/5065370293_9c7b36b5b9.jpg

http://farm5.static.flickr.com/4090/5065984702_52cce92a96.jpg

 

[ToadJones]

you could knee-brace the sidewalls to the top.

 

[RHTPE]

Knee braces could work. I would presume there are 2 beams supporting the rafters - why not make them deeper and try to develop a full moment connection between them at the vertical posts in each end wall? You could create a portal frame this way which should reduce the sidesway.


Ralph
Structures Consulting
Northeast USA

 

[azcats]

Shear panel between the side walls at the ridge line extending down near the top of the mailbox? You would lose "open-ness" of the structure but the community could use the area as a dry bulletin board if they wish.

Or you could make a wall that's flush with the front of the mailbox and extends base to roof. That could be a clean look.

 

[cslater]

I like azcats ideas, but I'm thinking they could get expensive, since you'd need to anchor the shear panels to the concrete somehow to make them work. Probably the least expensive solution is some type of knee brace, as was previously suggested.

Something like a 12" 4x4 with a lag bolt on either end might be enough. It's tricky though because you don't get a lot of capacity out of knee braces. The limiting factor (I think) tends to be splitting of the wood along the grain.

You might be able to do some sort of decorative/structural black steel knee braces too.

 

[fpr4]

How would you anchor the lag bolt between the brace and the column/beam?

 

[msquared48]

First of all, this screams that it wanted to be a pole type structure from the first, but that is water over the dam.

To make this work, I would add a central shear wall (with an opening for the mailbox) over and under the mailbox, forming an effective "I" shape of all the available shearwalls.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[delagina]

i LOL at these pictures. definitely not something i can brag with my friends specially if i cant make it work.

 

[fpr4]

who is bragging?

By the way, what's your brilliant input towards this solution other than your sophomoric slang?

 

[delagina]

dude, dont be so sensitive.

i'm sure you are a hundred times better engineer than i am.

i just LOL because it didnt occur to me that a mailbox shelter like that would still need a structural engineer.

cheers !

 

[BAretired]

I suggest a beam connected with a moment connection to each end wall.

BA

 

[ToadJones]

Like I said, cut some 4x4's on a 45º angle and counter-bore and lag screw them in to make knee-braces.
Knee braces really jsut make this a moment frame.
I do agree that it is a rather curious situation for an SE.