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Author Topic: How or is the deflection reduced with the use of braces?  (Read 7516 times)

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Offline funfunfun

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How or is the deflection reduced with the use of braces?
« on: February 03, 2011, 03:32:22 PM »
In designing my barn beams (girts) and calculating the load, an engineer told me that I can reduce the clear span by 1/2 the length of braces. Makes sense.   My question is that does this also apply to maximum load calculations on the beam's deflection? Or is the clear span for calculating defection only the length between the braces?  EG.  16' clear span beam with 3' X 3' braces at each end; is the clear span used for calculating deflection 13' ? or 10'? And is the total load over the whole 16' and width or only between the reduced clear span and width?  I have designed beams for load, aesthetics, ease of joinery and find that sheer is fine but deflection comes up too large occasionally; but as compared to old barns I looked at I am way oversize to them. Wood species will be eastern hemlock probably or eastern spruce as I have lots of trees. 

Offline Jim_Rogers

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Re: How or is the deflection reduced with the use of braces?
« Reply #1 on: February 03, 2011, 04:59:14 PM »
We we told probably more than once, that when an engineer looks at a frame and he calculates loads, he doesn't take into consideration any braces. What that means is the braces don't carry any load down to the post. They act as stiffeners to make the frame more ridged; so that when the wind blows on it from one side, it doesn't shift and rock from side to side.

While designing a frame for a customer, I was "running the numbers" through DonP's online calculators here on this forum and they (the timber sizes) were coming up as "failing".

The customer questioned how could that be. The frame was about the same as one of Jack Sobon's frame designs; either from his first book or second book.

To understand how it worked, I ran the numbers on that design (from his book). And his design failed.

I was stumped. Knowing Jack (as I have sold him tools, and taken his workshop), I called him up and point blank asked him how his frame worked and why my running of his numbers caused the beam sizing to fail.

He asked me what "span" was I using, and I told him. He said I was using the wrong span amount. We had a discussion about what span should be used, and why, exactly what you are asking.

He told me that he had a conversation with the head engineer at a very well know timber framing company, and he told Jack that he uses the span from one post to the opposite brace. So that's not post to post distance. Not brace to brace distance. But from left hand post to right hand brace, at the beam level.

If you don't understand what I mean, then let me know and I'll provide a drawing showing you what I mean.

Using this span, and running the numbers, the beams passed.

I don't know if that's what your engineer meant when he said "1/2 the length of the braces." But that would somewhat sound like it doing the same thing. If you take 1/2 of one brace length and 1/2 of the other brace length, you get one brace length..... Exactly what Jack told me he was using.

I hope that helps you.

Jim Rogers

PS. Welcome to the forum...
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Offline fred in montana

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Re: How or is the deflection reduced with the use of braces?
« Reply #2 on: February 04, 2011, 08:25:51 AM »
I am not suprised that some engineers ignore the braces in their beam analysis. It is a conservative assumption that also simplifies the calcs.

If they were to model the entire bent with the braces beams and posts, the stresses and deflections in the elements would be less that what they see under the assumptions noted above.

If you ask them to model it that way, they may do it. May cost more as it is a bit more work but with computer modelling it isn't hard.

The rule of thumb you and Jim talked about sounds like a good approximation, allowing you to utilize some contribution from the braces. It would be unconservative I think to say the span is the shorter distance between the braces.

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Offline funfunfun

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Re: How or is the deflection reduced with the use of braces?
« Reply #3 on: February 06, 2011, 03:45:14 PM »
I adjusted the load/ ft2 for the beam I want to use and recalculated using Benson and compared to the calculator and essentially got the same numbers except for deflection - the shear and bending moment are ok for both. If I only reduce the clear span the length by one brace then deflection is ok using both. Benson uses 1.6 EE6 for Modulus of Elasticity and the chart attached to the calculator uses 1.3 EE6 eastern red oak which makes a huge difference for deflection. I'll contact my building inspector and see what he says.

I'll do more research on how the brace effects bending moment and deflection and get back. 

Thanks for the response.

Offline Raphael

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Re: How or is the deflection reduced with the use of braces?
« Reply #4 on: February 10, 2011, 02:45:01 PM »
  Either way it won't fall down and I find as the bounce fades from my stride I find I like a little more of it in my floors.  ;) :D

  Early on I ran the numbers on Jack's frame design (with the spruce he'd verbally approved) and asked my engineer the same question...
  The half the brace formula only works for braces set at the conventional 45 angle where half of any vertical load on the brace translates to vertical load on the post, and the other half is outward thrust against it.  Braces pitched up (2' horizontal  x 3' vertical isn't uncommon) will pass more of that load down the post but resist wind wracking less.
... he was middle aged,
and the truth hit him like a man with no parachute.
 --Godley & Creme

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Offline fred in montana

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Re: How or is the deflection reduced with the use of braces?
« Reply #5 on: February 11, 2011, 10:01:17 PM »
I took a little time to model this in Visual Analysis which is a structural analysis software.

The idea was to consider a timber bent made up of two 8x8 columns with an 8x8 beam spanning from one to the other. The span length is 12 feet. There are 45 degree braces 3 feet in from the ends of the beam. The load on the beam is 250# per foot.

I modeled it 6 different ways, meaning I used different assumptions for each analysis.

1. model the beam as a 12' long simple beam (ends free to rotate under load)
2. assume the braces allow me to shorten the span and treat it as only 9' (still a simple beam)
3. assume the braces allow me to shorten the span to 6' (still a simple beam)
4. model the beam as a 9 foot beam again but this time assuming the ends are fixed against rotation.
5. model the entire bent as a whole, assuming columns have no external braces
6. model the entire bent as a whole, assuming there is some external support acting on the columns.

See this link for the various models and deflected shapes:
http://logdovetailjig.com/Bent%20analysis.pdf

The results are summarized below:

model          max bending moment(ft lbs)        deflection (feet)
1                     4715                                  .0199
2                     2652                                  .0063
3                     1178                                  .0012
4                     -1790                                 .0012
5                     1425                                  .0049
6                       488                                  .0006


Unless columns are externally braced, model # 5 is the most realistic and presumably most accurate model.

Model 2 uses the assumptions described in the previous posts. This assumption provides a fairly close approximation of the deflection ( over estimates it some but not alot).  It does overestimate the bending force by alot.

Disclaimer: This info is just given to illustrate some different assumptions  that can be made and how those affect the results, not to say that this design is a good design or not. Not only that but someone will probably spot an error in this somewhere!

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Offline funfunfun

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Re: How or is the deflection reduced with the use of braces?
« Reply #6 on: February 13, 2011, 12:08:05 PM »
Went looking for a calculator and found

http://courses.cit.cornell.edu/arch264/calculators/example8.1/index.html

I sent the following question:

In designing a timber frame barn I have reached a query in reference
to the uniform load of a beam when it is has 45 degree braces of equal
size at both ends.

1. Some say they make no load considerations and only add lateral
support.
2. Some say the clear span can be changed to the distance from one
support post to the beginning of the opposite brace.  It is this case
which some will agree to but will not give any real details. My
assumption is that if I reduce the clear span this way that I should
leave the total load without this reduction. This is a minor issue but
I would like some solution to this issue if possible.

Most importantly is the deflection of the beam. Usually, at least in
my case, the beam size must be large enough for the load will also  be
satisfactory for horizontal shear and fiber stress.  As the clear span
increases the deflection is increased.  My question is will a pair of
matching 45 degree braces supporting a beam (for lateral and some
load) somehow reduce the deflection calculations? And is this
reduction similar in the #2 as above as reducing the clear span?

Where can I find this information (or calculator) without getting too
technical.

I got the following reply:

There is no absolute answer, as the behavior of the system will depend on
the rigidity of the joints (difficult to know in timber construction), and
the relative stiffnesses of all structural members, including the posts. In
general, and as a rough approximation, such braces will reduce bending
stress, shear stress, and deflection in the portion of the beam between the
diagonal braces. As it is an indeterminate structural form, the actual
forces. Moments, and deflections can be best estimated using structural
analysis software. If you're ambitious, you can find free downloadable
software for PCs.

Thank you Fred in Montana.  Your number run and the above general response will allow me confidence in explaining to my local building inspector how beam sizes are adequate - other than rating the load over spec.

Thank You all.



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