Drag strut - do I need it here?

[GalileoG]

I have attached a sketch to this post. The question is in the sketch and it is about drag struts and whether or not I would need it for this case. Thank you in advance for the help.

 

[ToadJones]

I would think not, but it would help you transfer loads to bracing at location 4 and make the structure more torsionally stable.

 

[rb1957]

FWIW, i have trouble understanding the sketch ...
is this an L-shape room with supports/foundations at 4 pts ?

if the drag brace connects to the long side between 1 and 2, where'll the drag load go ? beam along the wall to 1 and 2 ??

is the drag brace trying to stabilize pt 4 ?

 

[SteelPE]

This is a tough question for me.

Load is attracted to stiffness, so if you have a brace along that line then that line is going to be very rigid. Members in line with the brace will want to attract load to the brace whether or not you tell them to. If you wanted to play it safe, figure the load applied to the brace and then distribute that load appropriated over the length of that line and design your members accordingly.

 

[ToadJones]

I read the sketch as being a diaphragm with either bracing or shear walls at the locations where the "X"'s are drawn.

By saying "...if I design the deck to span between 1 & 2..." I assume you mean you will design your diaphragm to span between 1 & 2 and not the decking itself.

 

[msquared48]

I would absolutely use a drag strut there. No question about it.

I would also place one 90 degrees to it at the re-entrant corner too.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[GalileoG]

Thank you for the responses.

I should have been more descriptive. That is an L-shaped building, and the 'x' represents cross bracing. Also, by 'deck' I really meant diaphragm.

SteelPE, I was thinking the same. Whether or not I design my diaphragm to span between 1 and 2, depending on the 'design depth' of that diaphragm segment, load will still try to make its way to brace 4, because there's stiffness there - and without a drag strut, there may be issues. Is this question often left to the judgement of the engineer or is there any guidance out there?

 

[hawkaz]

If this is a flexible diaphragm, you absolutely need a drag strut here.

 

[dhengr]

Show the direction of your joists and decking and the locations of your other major framing. I’m perplexed, you have lots of x-bracing and/or shear walls to react the lateral loads on the shorter wall faces, and two braces in one end wall line. But, none to accommodate the lateral loads on the longer exterior walls which must be supported by shorter lengths of shear walls or lesser x-bracing. Which is the most difficult orientation to design around here, as regards the roof diaphragm action, and how should the joist span, and where the bracing is placed?

 

[GalileoG]

dhengr,

Thanks for the response. The cross-bracing on the other orthogonal direction were not shown on for clarity (not because they don't exist) - My question is a theoretical one for lateral load applied along one direction (direction parallel to the drag strut.) I should have made a better job stating this in my original post. Thanks.

 

[JAE]

GallieoG - I would put drag struts in both directions off the re-entrant corner. Regardless of your design strength, the lateral forces will head towards the stiffer regions. Also - in a seismic condition, tying things together to work as a unit is a good idea - especially to avoid deck tearing at the re-entrant corner which is a natural stress-raiser point.

Direction of joists doesn't matter.