Lateral Bracing

Lateral Bracing

Lateral What? Exactly. Perhaps the most important aspect of your bridge and you aren’t sure what it is.

Lateral bracing is the term I use to refer to any pieces on a bridge that help keep the top chord from bending horizontally. In the figure before, lateral bracing is red:

Lateral Bracing

Why is lateral bracing so important?

As you may have read on this website that the shorter a piece of wood, the more compression it can hold. Lateral bracing serves to break the top chord into smaller sections, giving it more strength. Simply put, lateral bracing is the “truss” that goes on top of your bridge.

Why only on top?

The bottom member of a bridge is in tension, and is not going to bend or twist. Wood has the same tensile strength no matter how long it is. The top chord will want to bend and twist, and needs support to keep it straight.

Why an X?

The reason many builders choose X’s is because they make triangles, which resist deforming. Of course, it doesn’t have to be X’s. You could use half X’s in a zigzag pattern, just take away half of the pieces in the figure. Or you could use straight pieces, circles, or any combination. However, straight pieces would just make a rectangle, which won’t really help.

There is a place and time when you can use straight pieces for lateral bracing instead of X’s. That idea is illustrated by the Fernbank Bridge. Notice that this bridge uses an L-beam for the top chord. And a large L-beam at that. The key lies in that fact; the straight pieces across the top had a very large surface area joining them to the top chord. This automatically made them stiff, and able to resist deforming. This kind of made a triangle, so to speak.

How much lateral bracing?

The amount of lateral bracing you need to use is based on the ratio of dimensions of your top chord. You want to use only just enough and not overdue it in order to get the best efficiency. However, there is a danger of not using enough. As a general rule of thumb, I try and use the same of amount of bracing joints as I have truss joints to the top chord. You can see this idea in the example pictures.

There is another thing to consider. Let’s say your top chord is 5 units wide and 5 units tall (a square) That beam is going to be equally hard to bend in any direction. However, if your top chord is 2 units wide and 8 units tall, even though it has the same total mass as before, you will need to use more lateral bracing.

There comes a point when it is no longer useful to add lateral bracing. If your top chord is only 1 unit wide, and 9 units tall, your top beam is going to bend and twist like a slippery snake. There isn’t a lot you can do about it except increase the width.

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10 Responses to “Lateral Bracing”

  1. lamphead23 - October 2, 2008 at 9:29 pm

    thanks man! im doing a report on bridges. This will help alot!

  2. first timer - January 4, 2009 at 5:18 pm

    How do you make a cross/ X piece of wood without laying the pieces of wood on top of each other? Or is that what you’re supposed to do?

  3. Indra - February 13, 2009 at 3:24 am

    greetings…
    may i ask something question?
    this page says that ‘X’ lateral bracing is good.
    but why your fernbank bridge, the strongest bridge doesn’t use ‘X’ lateral bracing???
    thanks for reply to my email..
    many thanks…

  4. Sydney - May 12, 2009 at 1:00 pm

    this is super cool

  5. paul - October 7, 2009 at 9:54 am

    can u send me the study of lateral bracing i have a thesis about bridges..
    thx! hope you will response i need it soon..

  6. Wincy - January 13, 2010 at 11:02 am

    thanx, this helped my group’s project a lot

  7. Year8Girl - April 20, 2012 at 5:13 am

    Hello,

    I am in grade eight and my group has been given the task of building a truss bridge. However, we have no idea how to do so. If you could please make a post without all the scientific explanation for how to build a simple truss bridge that can hold up to 10 kg? This would be very highly appreciated and you would be the cause of a very high mark! :D

    -Sally

  8. Lydia - June 1, 2013 at 8:58 pm

    I am making a bridge in class out of only popsicle sticks and wood glue. I don’t quite understand the meaning of lateral bracing and truss, can you explain. Would it be better for my base to be made with popsicle sticks faced straight and the top with popsicle sticks faced diagonal or like triangles.

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