Cross Bracing And It's Effect On Span Distance? - Building & Construction - DIY Chatroom Home Improvement Forum

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07-01-2011, 07:35 AM   #1
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## Cross bracing and it's effect on span distance?

Out of curiosity, if you have a deck beam span of 10' for instance, and you add cross bracing parallel to that beam back, say 3' (let's assume a 6x6 post is supporting the beam, and 4x6s are used for the cross braces), what affect does that have on the theoretical beam span? I asked our local codes department, and they couldn't find it in their literature, but gave me some ballpark estimate of reducing the span by 30%. Any input would be appreciated. Thanks!

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07-01-2011, 09:11 AM   #2
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Can you provide some kind of sketch. Your description is a little confusing.

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 07-01-2011, 09:38 AM #3 Civil Engineer   Join Date: Mar 2009 Location: Boston Posts: 5,688 Rewards Points: 4,956 If the bracing is parallel to the beam, it isn't cross bracing. I don't know what it is, this is a very confusing description. Cross bracing by definition is bracing applied between two structural elements, i.e. joists, at an angle to the structural elements. If it is at approximately 45 degrees, it is commonly referred to as X bracing. Cross bracing serves several purposes. First, it connects two structural elements, and effectively increases the moment of inertia in the plane of the bracing. This increases the lateral stiffness and strength of the structure. It has no effect on the span length, which by definition is the unsupported length of a structural element between two supports. Cross bracing on long, slender columns has the effect of reducing the effective length of the column. This only applies to vertical structural elements (columns), this discussion is not applicable to horizontal beams, which rarely act as columns, at least in residential applications. Cross bracing on horizontal elements has the effect of reducing the effective length of the beam in the horizontal plane of the cross bracing, which reduces the effective length of the beam for purposes of lateral buckling analysis. This is important in bridge applications, where large lateral loads on slender I beam elements can and do routinely occur, and lateral buckling is a potential problem. I have never seen a case of lateral buckling in residential applications, however with the increased use of I joists it is a theoretical possibility, and there are guidelines published by manufacturers of I joist products regarding the need for cross bracing, which typically must be installed before allowing workers to walk on I joists.
 The Following User Says Thank You to Daniel Holzman For This Useful Post: tpolk (07-01-2011)

07-01-2011, 09:50 AM   #4
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Quote:
 Originally Posted by Daniel Holzman If the bracing is parallel to the beam, it isn't cross bracing. I don't know what it is, this is a very confusing description. Cross bracing by definition is bracing applied between two structural elements, i.e. joists, at an angle to the structural elements. If it is at approximately 45 degrees, it is commonly referred to as X bracing. Cross bracing serves several purposes. First, it connects two structural elements, and effectively increases the moment of inertia in the plane of the bracing. This increases the lateral stiffness and strength of the structure. It has no effect on the span length, which by definition is the unsupported length of a structural element between two supports. Cross bracing on long, slender columns has the effect of reducing the effective length of the column. This only applies to vertical structural elements (columns), this discussion is not applicable to horizontal beams, which rarely act as columns, at least in residential applications. Cross bracing on horizontal elements has the effect of reducing the effective length of the beam in the horizontal plane of the cross bracing, which reduces the effective length of the beam for purposes of lateral buckling analysis. This is important in bridge applications, where large lateral loads on slender I beam elements can and do routinely occur, and lateral buckling is a potential problem. I have never seen a case of lateral buckling in residential applications, however with the increased use of I joists it is a theoretical possibility, and there are guidelines published by manufacturers of I joist products regarding the need for cross bracing, which typically must be installed before allowing workers to walk on I joists.

Excellent info!! But I need to take an aspirin.

07-01-2011, 01:32 PM   #5
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Quote:
 Originally Posted by Daniel Holzman If the bracing is parallel to the beam, it isn't cross bracing. I don't know what it is, this is a very confusing description. Cross bracing by definition is bracing applied between two structural elements, i.e. joists, at an angle to the structural elements. If it is at approximately 45 degrees, it is commonly referred to as X bracing. Cross bracing serves several purposes. First, it connects two structural elements, and effectively increases the moment of inertia in the plane of the bracing. This increases the lateral stiffness and strength of the structure. It has no effect on the span length, which by definition is the unsupported length of a structural element between two supports. Cross bracing on long, slender columns has the effect of reducing the effective length of the column. This only applies to vertical structural elements (columns), this discussion is not applicable to horizontal beams, which rarely act as columns, at least in residential applications. Cross bracing on horizontal elements has the effect of reducing the effective length of the beam in the horizontal plane of the cross bracing, which reduces the effective length of the beam for purposes of lateral buckling analysis. This is important in bridge applications, where large lateral loads on slender I beam elements can and do routinely occur, and lateral buckling is a potential problem. I have never seen a case of lateral buckling in residential applications, however with the increased use of I joists it is a theoretical possibility, and there are guidelines published by manufacturers of I joist products regarding the need for cross bracing, which typically must be installed before allowing workers to walk on I joists.

Here's a graphic illustrating my question. So if a deck beam has a support tied to the post it is resting on and the beam itself at a 45 degree angle, it does nothing to the span distance from the post to the next post? So, say you could only span X feet with your give beam setup, you couldn't increase that span distance by adding bracing PARALLEL to the beam itself?

 07-01-2011, 01:34 PM #6 Tileguy     Join Date: Mar 2006 Posts: 10,705 Rewards Points: 558 Wouldn't that be a "knee brace" and not cross-bracing? I also don't see a deck beam anywhere. __________________ XXX
07-01-2011, 01:53 PM   #7
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Quote:
 Originally Posted by Bud Cline Wouldn't that be a "knee brace" and not cross-bracing?
I suppose so. Terminology aside, what I'm depicting is the question at hand.

Quote:
 Originally Posted by Bud Cline I also don't see a deck beam anywhere.
Really? Clearly labeled in the drawing.

Last edited by mgh-pa; 07-01-2011 at 01:58 PM.

 07-01-2011, 04:27 PM #8 Member     Join Date: Jul 2011 Posts: 1,378 Rewards Points: 500 I can't imagine that will give you much benefit or increase on the beam span. That type of bracing is usually used for lateral stability and not for compression loading. The brace will take some load from the beam and transfer it to the column, but the load it can withstand will be minimal. You will be providing excessive lateral loads on the column that is not sized to take it. In order for it to truly increase the span you would need to tie the footings in and reinforce the columns. This increase in structural work on the columns and footings is likely equal to the cost increase of up-sizing the beam. I attached the image to show the direction of the forces. The purple arrows indicate the laterla loading on the column that you would need to address to accommodate the larger span loads. Last edited by AGWhitehouse; 06-19-2013 at 02:59 PM.
 07-01-2011, 04:41 PM #9 Member     Join Date: Mar 2010 Location: Kansas City/Oregon Coast Posts: 10,312 Rewards Points: 1,022 There’s like 5 threads on this same deck. All that does is create confusion and helps nobody, especially OP. If someone wants to offer help they should search out and read all of the threads to get a clear picture of what’s going on, but who wants to do that. __________________ A Picture Is Worth A Thousand WordsEspecially In The DIY Chatroom
07-01-2011, 06:16 PM   #10
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Quote:
 Originally Posted by AGWhitehouse I can't imagine that will give you much benefit or increase on the beam span. That type of bracing is usually used for lateral stability and not for compression loading. The brace will take some load from the beam and transfer it to the column, but the load it can withstand will be minimal. You will be providing excessive lateral loads on the column that is not sized to take it. In order for it to truly increase the span you would need to tie the footings in and reinforce the columns. This increase in structural work on the columns and footings is likely equal to the cost increase of up-sizing the beam. I attached the image to show the direction of the forces. The purple arrows indicate the laterla loading on the column that you would need to address to accommodate the larger span loads.
Thank you! Exactly the information I was looking for. Thanks for the diagram as well. This pretty much confirmed that our local building department may not be truly up to snuff on all things structural. I wanted a second opinion and some clarification, and this helps for sure. I'm either going to up size the beam or add an addition support post to reduce the unsupported span.

Quote:
 There’s like 5 threads on this same deck. All that does is create confusion and helps nobody, especially OP. If someone wants to offer help they should search out and read all of the threads to get a clear picture of what’s going on, but who wants to do that.
The deck plan has been totally revamped since that original post, new drawings have been made and submitted to the local codes department (as per the suggestions in the first threads), so those earlier threads have no pertinence to what's being asked with regard to this design, hence the new thread.

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Last edited by mgh-pa; 07-01-2011 at 06:45 PM.

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