beam sizing question for barn

[Wisconsintimber]

I am trying to make a plan for a post and beam type barn/shop.  Similar to timber framed but held together with plates and bolts vs mortise and tennon(I would love to do a mortise and tennon, but with a lack of time and the learning curve, not sure I could pull it off) 
So the size of the center section would be 20' x 30'long with a 12' lean on each side.  There would be 4 bents, 10' apart(3 bays) and they are 20' wide.  I am wanting a loft up above for an office or whatever, not for heavy equipment.  I am wondering if my cross beams can span the 20' and support the loft with out a post in the center?  I plan on using white pine.  What size would the beams need to be?  8x10? 8x12?  I am hoping to keep the center open without posts for a shop.  Or do I need to change my idea all together?  A pole shed would probably be faster to build, but I would like something different...

Thanks for any ideas

[Roger Nair]

On the menu at the top of the page click on extras and then on the submenu tool box, in the tool box you will find a uniform beam calculator with a drop down species list.  Use 40lbs/sq ft combined load and 10 lbs/sq ft dead load.  You will find that you will need a very large stick.  An intermediate post will be looking like an easier path.

[Carpenter]

A 40/10 load is pretty standard for a floor in a house.  The way I understand that it is 40#s live load and 10#s dead load, so 50#s total load.  So, if your beam is spanning 20' and your bents are 10' apart your tie beams are supporting 200 sq' of floor, x 50 = 10,000 #s total load.  You may want to add up the weight of the beam, joists, subfloor, and flooring that will be supported by that beam, but that would be the dead load.  Typically in light framing the dead load is figured in at 10#s per square foot.  With timber framing it does vary a little bit depending on how the structure is designed but that does give you a starting point for some load calculations. 

So, go to the forestry forum tool box and select Don P's calc's for a uniformly loaded beam.  I usually use the drop down species for convenience.  Put in your total load at 10,000#s and your dead load at 2000#s unless you add up the dead load and find that it is different.  Put in the width and depth of timber, the length in inches, species, grade and hit the calculate button and it will tell you whether or not the timber will pass.  It even gives the timbers projected deflection.  If the timber does not pass you can beef it up or use a different grade of timber.  I think it's a really useful tool.  I'm sure others could explain how to use it better than I could. 

[USredneck]

Hello and thanx to the administrators for letting me in so fast.

I have always been bugged by the question of whether the standard beam braces contribute to the beam load-carrying characteristics (strength and deflection). In the case described by Wisconsintimber, one would assume that the post-to-beam braces would come in at about 3 feet from each side of the beam, cutting down the totally free span to 14 feet. Now, I am not claiming that the 45 degree braces have the same effect in supporting the beam load as two additional vertical posts at the same 3 feet from the wall posts would, but isn't in unfair - though of course on the conservative side - to not be taking into account the braces at all when we calculate beam spans based on the standard beam geometry parameters (section modulus, length etc) ?

When one uses bracing from above, as in the case of attic room trusses, one DOES take the effect of the braces into account. Why are we - the timber framers community - selling ourselves short when we could be adding feet of free space under the same beam or using smaller section beams while meeting code requirements ?

Does anyone know of any easy to use beam sizing calculators which take into account the braces ? I know that a full finite elements program would do this, but it is a pain to set up the geometry, while a quick formula based calculation, even trimmed on the conservative side, would make life much easier at the conceptual design phase, while the final calculation for the permit bureaucracy would still be done by a structural engineer anyways.

Eager to see opinions or info on this.

[beenthere]

Welcome to the Forestry Forum.

Interesting forum name for someone in Moscow, Russia.

Also interesting question... regards the braces being considered in the design or not.

[USredneck]

Yeah, defectors go both ways I suppose !

[Roger Nair]

Hey Red, the way I think about braces is that a loaded brace has destructive potential by directly translating sag in the beam into horizontal thrust.  The post will deflect but more seriously the post to beam joint will have a strong steady tension force, the normal joint styles are not strong against tension.  Furthermore the loaded post can act as a lever applying additional force to the joint.  Therefore normal practice is to disregard braces as a means to reduce span.  So there still will be deflection in the beam, but frame will season, the post will shrink but hopefully in a balanced way where shrinkage and deflection remain in the long run balanced, only if the beam is solely suited to carry the entire load.

[Jim_Rogers]

What Roger said times two.

However, some engineers, name hidden to protect the guilty, reduce the span by one brace leg length. So if you had two posts 12' apart and from those post you have two 3' braces then they may consider the span to be 9' instead of 12'.
The would not consider it 6' as that would mean that a huge amount of the load would be coming down the brace to the post. And it could deflect the post as Roger has stated.
Braces are suppose to stiffen the frame from racking side to side, not support the load of the beam it is connected to.

I was also told by an experienced timber framing engineer about the 10% rule.
This is if a beam was tested in a spreadsheet beam sizing calculator (something similar to the one in the red tool box here on this forum) and it failed by less than 10% of the total allowable value, that the would consider it as strong enough. And that it did or would pass.

I try to not use this 10% over value rule but it is nice to know that sometimes you may need or could use it.

Jim Rogers

[USredneck]

It is 6:30 AM over here and just woke up for a quick nature call, but can't help from answering to your posts: for one, I think Roger's comment only makes sense if one is talking about braces hanging from above on a knee wall situation, otherwise I can't see where the tension would come from at the post-to-brace joint, and what the meaning of "loaded post" would be other than the roof rafters lateral push on the upper portion of the post. Even then, the top portion of the post only being supported by the rafters in compression when the beam deflection gets transmitted to the post, could put the post at risk of fracture if excess inwards bending torque results at the post-to-brace joint because of beam deflection, assuming the joint does not fail first. This is not the case for the brace-from-below geometry as the post is presumably fixed to the sill in one way or another, providing additional resistance to the outwards bending torque applied at the brace-to-post joint.

I must say that, given the obvious tension loading of both brace joints (brace to beam and brace to post), I would not in my wildest fantasy consider the upwards bracing as a candidate for beam support, unless the brace was connected through thick metal gusset plates to both the beam and the post.

On the other hand, both the "one brace length" and the 10% rule of thumb referred to by Jim, would seem to relate to the more standard brace-from-below picture I had in mind when I posted my question. But Jim stated that he seconds and doubles Roger's comment, so I am coming more confused out of your kind attempts to assist me in my inquiry.

In any case, you may want to explicitly comment on the brace-from-below geometry, as there is no tension loading of the joints in a standard beam deflection situation and the brace would seem to act directly in supporting a 45 degree component of the vertical load at the brace-to-beam beam point (times cosine(45)).

I would like to point out that when considering large spans, of the order of 18-20 feet or even more as in one of my contraptions, the required beam sections, if one assumes no assistance from the braces, become so large that the dead load on the beam makes it sag significantly before even any useful load is applied on it. Therefore using something like the "one brace length" shortening of the beam span used in the calculation, is useful in more than one way by reducing the required beam sections and their self weight.

I've been thinking about using an attic truss kind of geometry with heavier than usual rafters and metal or plywood gusset plates to make good tension joints as a solution to this problem, assuming full second floor live loading which is my aim from the beginning regarding this question, but it would reduce the available knee space above. Another solution would be a hybrid frame with a metal 3-d truss instead of a beam between the posts. I know it might sound sacrilegious in this forum but picturing it in my mind looks in fact good, especially if one can achieve the right dark or even shiny color for the metal. Wooden trusses with galvanized plates would look awful, but one could possibly hide them with some kind of thin cladding with veneer or plywood or some other sheet like material. But any visible seams would make the "beam" look kind of funny.

Just tossing some thoughts around.

Thanks many for the responses.

Going back to sleep for a while more.

[Roger Nair]

Red, we have a basic misunderstanding.  Let's keep focused, no rafters, no kneewalls, no top side braces.  Just this 20 ft beam with a post at each end.  We establish the load, do some calcs and find that we will need a 8 x 16 beam.  All the old buildings in our area has 8 x 10 beams, so I want an 8 x 10.  So I reason if I put a 3 ft. brace under the beam at each end, I will cut the span to 14 ft.  What happens?  Loading from the beam is taken by the compressed brace and delivered to the post in vertical and horizontal components in proportion to the slope simultaneously pressing down and pushing out on the post.  The post reacts by deflecting outwards but force remains in the system, the frame has to resist the outward force at the post foot and post top at the beam connection with the force in proportion to the post length and brace length.  As you might expect, the shorter the brace the greater the force at the post-beam connection.  The load at the post-beam joint, applied by a compressed brace, is in tension.  A steady floor load will result in a steady tension load, not a good situation.

 

[USredneck]

"The load at the post-beam joint, applied by a compressed brace, is in tension. "

Roger, thanks for all the effort to explain things to me, but I agree, there is definitely a misunderstanding: we have a situation like inside a nutcracker, with two arms joined at the end in free pivot and the brace being the nut being crushed by the force my hand is placing on the two nutcracker arms, trying to bring them together.

I fail to see any tension loading anywhere on the points where the nut shell meets the nutcracker. The brace is being crushed by the combined gravity load which deflects the beam (one nutcracker arm) and the resistance the post places to the brace push against the post. In fact, if the brace was sawed perpendicularly to its length into a series of solid pieces without any adhesion to each other and therefore complete lack of resistance to tension, it would still function adequately - assuming no external lateral loads on the posts and only a gravity load on the beam.

The brace ends compress against the mortised "nest" on the post and the beam. Where is the tension ? the pegs would almost not participate at all. The post experiences only compression at the "nest" where it meets the brace, and, yes, the opposite (outer) side of the post bending outwards will experience tension, but I am sure you are not referring to that tension.

[Roger Nair]

What I am referring to is reaction force.  The nutcracker's hinge is in tension while cracking a nut and the post-beam joint experiences tension as the brace is compressed, both are examples of reaction in a lever system.  So back to the example of a building and long duration loading, beams that are not adequate is size can experience plastic deformation in service.  In otherwords, the deflection can reform the beam with a permanent set, remove the load the beam still sags and does not return to the original position.  In effect a new set point in the beam is created relative to position.  The load is re-applied and the beam deflects but the deflection is in addition along with the sag.  This is a building in a state of slow collapse.  I have spent 35 years working on old buildings, plastic deformation is real and joints do open up, pegs fail, posts are riven by braces pushing while joints resist the outward thrust.  Think long term effects, what will or can happen in a century.  This is how I view the brace effect in building with weak beams and long spans.

[USredneck]

Roger,

only now I realized you were talking about the post-beam joint !!
Yes, I can see why tension would develop there, though it seems to me that only a small component would be along the beam axis, the main one being upward thrust resisted by the mortize-tenon combo and the standard nesting of the whole beam in the post. I have not associated it with the braces,
I get it now, you are not recommending smaller section beams because they would deform more at the brace-beam joint.
Again, gusset plates at the post to beam outer surface should help resist the tension there. I do have a heavy 8x10 beam that is too long for its own weight receding from its post nesting, it may be related to the effect you are describing. I thought it was due to insufficient rafter bracing which resulted in the knee-wall section of the post moving outwards, and have been thinking about ways to jack it back into place and add metal gusset plates to keep the joint from falling off the post recess and relying solely on the 2" tenon inside the post for support.
The effect you are describing must be related to why concrete beams and floors have a component of their steel reinforcement web closer to the top surface near the edges, when they are not free floating but fixed in the surrounding concrete frame. But again, this is designed to resist the tension at the top surface that is the result of upward bending, similar to the upward bending a timber beam experiences from the brace action as you described. No significant horizontal tension component at the joint would seem to arise, intuitively thinking.

Thanks so much for clearing things up for me !!

[classicadirondack]

seems to me this thread has been hijacked!

[jimdad07]

Looking at it through newbie timber framing eyes you would think that if the beam sags in the middle that brace would turn into a fulcrum point where it goes into the tie beam and would create some thrust against the wall post and lift where the tie beam tenon goes into the wall post.  Am I seeing it correctly?

[USredneck]

jimdad07

That's what I said, basically, but Roger sees tension at that beam-to-post joint, which could only happen if there was a horizontal pulling force along the beam at that joint. I see mainly upward thrust, as do you. Since the beam sits in its own nest at the post and the tenon is typically 2" thick, the pegs should not be feeling much in terms of tension related shear forces if the force created on the beam-post joint by the brace is mainly pointing upwards. This should hold true even when the timbers dry up and things move around or away from their initial dimensions.

[jimdad07]

jimdad07

That's what I said, basically, but Roger sees tension at that beam-to-post joint, which could only happen if there was a horizontal pulling force along the beam at that joint. I see mainly upward thrust, as do you. Since the beam sits in its own nest at the post and the tenon is typically 2" thick, the pegs should not be feeling much in terms of tension related shear forces if the force created on the beam-post joint by the brace is mainly pointing upwards. This should hold true even when the timbers dry up and things move around or away from their initial dimensions.

I can see where the tension is coming from too.  If you push down on the brace then it will push the bottom towards the outside post, it has nowhere else to go.  Its not like a post that's vertical and carries load straight down to the foundation.  From what I know of building you have to take into account all of the loads and that means you would also have to think of the rafter thrust pushing out against the top plate which is also putting your tie beam to wall post joint under tension.  I'm not an engineer so I may be way off base here but that's how it appears to me.

[Roger Nair]

Red, I'll give it one more try.  You wrote:

"That's what I said, basically, but Roger sees tension at that beam-to-post joint, which could only happen if there was a horizontal pulling force along the beam at that joint."

The brace is pushing against both the post and the beam.  Any text on structures will have the topic of force systems and various methods to visualize and calculate forces in the opening chapters.  I however will ask you to do a thought experiment.  Mount a bicycle seat on a stout stick.  Foot the stick where the wall meets the floor and incline the stick 45 degrees.  Adjust the seat and sit on the seat.  The inclined stick will push you away from the wall as you fall towards the floor.  Now if you fasten hand holds to the wall and grab on, you could maintain position only by holding on, the tension force is steady and if your grip fails, you fall.  As you experiment with different slopes the relationship between slope of the stick is directly proportional to the tension in your arms and grip.

Although my example is poor, it illustrates two things.   An inclined member will redirect force and a static vertical compression load can result in a horizontal tension load.

You wrote:

" Since the beam sits in its own nest at the post and the tenon is typically 2" thick, the pegs should not be feeling much in terms of tension related shear forces if the force created on the beam-post joint by the brace is mainly pointing upwards"

Tension at the joint should not be minimized by tenon thickness or peg diameter, the whole issue is to design the beams adequate strength.  In general, braces normally are at 45 degrees, thus  the resultant vert. and hor. forces are equal.  You may want to disregard this but you need to take a closer deeper look. 

[USredneck]

Roger, I see your point, and you may well be right but something does not sit quite right with me in your example. I am not sure about the bicycle setup where you have rotational and translational degrees of freedom absent in the fixed brace setup, but at the braced post-beam geometry we do not have symmetric loading on the two arms of the pivoting joint, which we do have (symmetric loading) in the case of the nutcracker, where in addition the angle of operation is much smaller than 90 degrees. With the brace at 45 degrees and only a vertical load on the (horizontal) beam, that is a load parallel to the post, I fail intuitively to see a horizontal force pulling the beam out of the post. I will have to think about it, sit down and draw some forces, and get back to you.

jimdad07

I think you are concentrating on the wrong joint - the joint under tension according to Roger is the post-to-beam joint, not the brace-to-post bottom joint, where I can't see any tension (pulling apart action).

[jimdad07]

Roger, I see your point, and you may well be right but something does not sit quite right with me in your example. I am not sure about the bicycle setup where you have rotational and translational degrees of freedom absent in the fixed brace setup, but at the braced post-beam geometry we do not have symmetric loading on the two arms of the pivoting joint, which we do have (symmetric loading) in the case of the nutcracker, where in addition the angle of operation is much smaller than 90 degrees. With the brace at 45 degrees and only a vertical load on the (horizontal) beam, that is a load parallel to the post, I fail intuitively to see a horizontal force pulling the beam out of the post. I will have to think about it, sit down and draw some forces, and get back to you.

jimdad07

I think you are concentrating on the wrong joint - the joint under tension according to Roger is the post-to-beam joint, not the brace-to-post bottom joint, where I can't see any tension (pulling apart action).

I was talking about the post to beam joint, the brace joint is in COMPRESSION.  I was talking about the brace adding to the TENSION  on the post to beam joint because of the bottom of the brace pushing out possibly because of a sagging beam, if the beam sags it's going to put more COMPRESSION on the brace and could possibly push out against the post where the brace goes into its mortice.  Again, I could be wrong but that's just how it appears to me.

[USredneck]

jimdad07

OK, got it, your explanation makes more sense than Roger's to me, though I bet you it is the same effect .. somehow.

[Heartwood]

Hi all,
To muddy the waters more and complete this vicious circular argument, keep in mind that load goes to stiffness, so if the brace is cut perfectly then it will pick up the bending load whether we want it to or not. The logical conclusion would be to make the brace sloppy so it only goes to work once the frame deflects from lateral loads. Not that any of us do that.

[beenthere]

Heartwood

vicious circular argument

:
I read into this thread a discussion, but nothing vicious, nor at the argument level.

We can all learn from a discussion, what appears to some and not to others.

[Heartwood]

Sorry, I wasn't talking about this discussion, but rather this conundrum (as in "viciuos circle") that has been wrestled with for a long time among timber frame designers.

[Den-Den]

Does anyone know of any easy to use beam sizing calculators which take into account the braces ? I know that a full finite elements program would do this, but it is a pain to set up the geometry, while a quick formula based calculation, even trimmed on the conservative side, would make life much easier at the conceptual design phase, while the final calculation for the permit bureaucracy would still be done by a structural engineer anyways.

Eager to see opinions or info on this.

Beam sizing calculators need to be inherently conservative because many times they are used instead of a "final calculation by a structural engineer".

Diagonal braces certainly do more than resist racking (if they fit tightly), adding some vertical support and making a simple beam act a little bit like a continuous beam with multiple supports.  They also add some forces that have to be resisted by the assembly.  Trying to take advantage of the positive effects without considering the negative effects would result in a frame not be as strong as expected.  Would it fail?  Maybe or maybe not; a full finite analysis and consideration for the stresses at each joint vs wood quality and joint quality is necessary when we try to stretch the limits.