Need some help on beams

[tsodak]

This is not really a timber framing question but is kind of a hybrid....

I just bought my first mill after about a year of looing around (a norwood with two extensions) and plan two major projects in the next year.  First and primary is a deck. We just built last year, and I need to add a 12 or 14X60foot deck along the 60 foot second story side of my home. Because this is above the walk out windows and doors, it needs to be wide and narrow, so likely will be 12 feet. Here is my plan, please shoot holes in it for me.

I plan to dig the footing holes 5 feet down, (below frost) and then mount the posts on top of this pier above the lower porch floor and inch for moisture control. I have already cut and dressed raw Eastern Red Cedar logs 10 feet long in the round with my hand power planer to use as the posts. They run from 20 inches at base to 10 inches at the top where the crossing beam will mount. I intend to use a 4x4 or larger angled brace to both help secure the beam to the poles and to brace the beam. I intend to cantilever the posts and main beam two feet in from the side of the deck. obviously the beam will need to be 60 feet long as well. I am hoping to use all cedar.

Here is wehre I start to get a little hazy.

Since we spend time in the basement walkout a lot, I would liek the view to be as unobstructed as possible. What i would like to do is span 20 feet so I could do it with four poles. This means each span is 20 feet. To carry that long distance I am thinking I would use a beam constructed of  2  6x6 cedars heart cut and bolted on top of each other to create on unified beam 60 feet long. Then I would run 2x8's on 2 foot centers from the top of this beam over to the mounting plate on the house. I was thinking I would use 2 inch thick cedar as the decking in random widths and lengths. Alternately I could use the 2x8's on 16" centers with 1 inch stuff if it were going to be to heavy.

Any thoughts would be greatly appreciated. this is a learning process for the later barn which is being planned.  I found a HUGE pile of 18-24" gren ash and a few burr oak all bucked to 6-10 feet today all piled up and drying. Best yet they are free for the taking, unless his boss says no, fingers crossed.  I was thinking hard about changing and using the ash as decking as I could really knock that out fast compared to the cedar, which is all standing dead and I have to cut and clean it.  But some things I read here make me nervous about using ash in a horizontal application outside even if I were to use water seal or the equivalent annually.

Any thoughts on that???

Finally, are powder post beetles a universal problem??? Here in SD I am a half mile away from my nearest tree, so risk of infection is low from other trees....  are they basically already infested from anywhere, or is it more prevalent in certain areas?? Do I need to worry about cutting and using that ash or cottonwood (populus deltoides) as framing lumber for my barn, kiln, saw shed, greenhouse, playhouse, hmmmmm, what else can I find to cut wood for????

Thanks for your help on some really wide ranging questions.

Tom

[Don P]

Stacking 2 beams to make one doesn't give much strength unless you go to split ring connectors or some type of shear fastener. The 2 beams will each act as a single 6x6, slipping past one another unless they are very firmly attached to each other. Don't build beams by stacking wood flatways. You can build up thinner full depth pieces, for instance a carpenter making a 3 ply 2x12 girder.

The 20' span is a doozy, and in eastern red cedar. It's not listed in the NDS tables, looking at the Wood Handbook and making some comparisons, I would use white pine's allowable design values. That is a leap in logic that isn't allowed, so you have to satisfy yourself.

I'm assuming you are 8' from the house ledger to the beam and then 2 more feet overhanging it. The "tributary load" on the beam is half the 8' (half of that span bears on the house ledger) plus the 2' overhanging. So 6'x20'=120 square feet. I'm assuming a 50 lb per square foot combined load on the deck, that is as light as I would go, 120x50=6000 lbs on the beam. I got it to pass at 10x14. I'm thinking closer posts. A 7x9 beam worked at 10' spacing.

Deck collapses injure and kill more people every year than all weather and seismic events combined. Usually family and friends, the latest I heard about was at the house after a funeral. Be conservative  ;).

I wouldn't use ash in any outdoor projects. Well, canoes, baseball bats... but its a better indoor wood. You shouldn't have powderpost, until you bring them home  . It sounds like you're in a lot lower risk zone than me though.

[tsodak]

I hear  what you are saying on the beam stack, but if I were to bolt them every 5 feet with a half inch carriage bolt through from top to bottom that would stop the side slip right???? Really for all that matters, I could do the same with three 4x8's stacked . That would actually be more efficient for me to get out of the logs I have. Then at each lap joint in any of the laminates run a bolt though and crank her down. Thoughts???

[Don P]

This is a link to a page describing historical use of Keyed Compound Beams

That page refers to Kidder, that is the engineering manual Raphael recently downloaded. In it Kidder describes built up beams of vertically fastened dimensions first. He correctly states that they are stronger than solid sawn beams. He then goes on to describe the compound beam you are talking about. I can't open this but hopefully its my connection. It starts on page 607.

Kidder

If that doesn't work and you'd like a copy, pm me your snail mail and I'd be happy to mail one.

[Don P]

Let's see if this section from Kidders manual worked  ???

Built-up Wooden Beams.
Wooden beams or girders built up of planks, spiked or bolted together side by side, will generally be somewhat stronger than a solid beam of the same dimensions, because the planks will be better seasoned and more free from check cracks and other defects. For beams or girders 10 ins. or less in depth spikes will usually be sufficient for binding the planks together, but for deeper beams bolts should be used in addition to the spikes, to prevent the planks from separating and the outer planks from warping or curling away from the
others. Two bolts should be placed at each end of the beam and about four feet apart between. When beams are built up in this way each plank should be the full length of the beam, or, in the case of a continuous beam, the planks should break joint over the supports Built up beams should always be set with the planks on edge, and never flatways.


Compound Wooden Girders.
It is often desirable to use a wooden girder for a longer span or greater load than would
be safe for the deepest single beam that can be obtained, or for a beam built up of planks. In such cases compound wooden beams may be used. By a compound wooden beam or girder is meant a beam built up by placing two or more single beams one on top of the other, with the view of having them act as a single beam having the depth of the combined beams. Thus if two 10 X 10-inch beams were placed one on top of the other, and the upper one loaded at the centre, the beams would act as two separate beams (Fig. 1) and their combined strength would be no greater than if the two beams were placed side by side.


If, however, the two beams can be joined so that the fibres of the lower beam will be extended as much as would be the case in a single beam of the same depth or in other words, so that the two beams will not slip on each other, the compound beam will
have four times the strength of the single beam. Various attempts have been made to join beams thus placed so as to prevent the two parts slipping on each. Probably the most common form of compound beam, as used in American building construction, is that shown in Fig. 2,


diagonal boards in opposite directions being nailed to each side of the two timbers to prevent their slipping on each other. Prof. Kidwell made nine tests of this style of beam, six having a ratio of span to depth of beam as 12 to 1, and three as 24 to 1. The shorter beams gave an average efficiency without much variation, of 71.4 per cent., and the longer beams an efficiency , of 80.7. It was found that the beams failed by the splitting of the diagonal pieces or the drawing of the nails "in every case, long before the beam broke, the
struts split open or the nails were drawn partly out, or bent over in the wood, thereby permitting the component beams to slide on each other. It was found that no amount of nailing could prevent this." When built with diagonal boards 1 inches thick, nailed with 10 d 's as in Fig. 2, the working strength of such a beam may be taken at 65 per cent, of the
strength of a solid beam of the same depth, and of a breadth equal to the breadth of the timbers. The deflection of the beam, however, will be about double that of a solid beam of the same size, and on that account this style of beam is not to be recommended for  supporting floors with plastered ceilings or carrying plastered partitions.

Keyed Beams.
Prof. Kidwell also tested several styles of keyed beams, with the result that a compound  beam keyed and bolted together, as shown in Fig. 3, was found to be the most efficient form that it is practicable to build. It was found that with oak keys it was possible to obtain
an efficiency for spruce beams of 95 per cent., while the deflection varied from 20 to 25 per
cent, more than would be expected in a solid beam. By using cast-iron keys the deflection was found to be but little, if any more, than with a solid beam. The keys must be wedge-shaped, as shown in Fig. 4, so that they can be driven tightly against the end wood.
Prof. Kidwell recommends that for ordinary purposes an efficiency of 75 per cent, be allowed when oak keys are used and 80 per cent, when the keys are of cast iron. The width of oak
keys should be twice the height of the key. Numerous small keys closely spaced gave better results than fewer large keys. In the centre of the span a space equal to about one-quarter of the length of the beam should be left free of keys, bolts, etc.

[tsodak]

That is a great page!!!!

I have to think on this a little more, but I think I can come up with materials and design to create a laminate beam that would meet all of the above. I have to see what kinds of materials come out of the mill after I get it set up this weekend!!!

If anyone else has thoughts I would love to hear them!!

Thanks

Tom

[Don P]

Scroll down the pdf file below to "split ring connectors". That is the "modern" way of doing the same work as the keys. That might be another direction to look at.
http://www.clevelandsteel.com/divisions/construction/PDF/Timber_Conn_print.pdf

A flitch plate beam is another composite beam worth thinking about. Its a sandwich of wood, steel plate, and wood, standing vertically and bolted through. I beam can hold wood fillers too.

There was more on the keyed beam that had more detail than I can shrink to fit here, if you go that route pm your email and I'll send some more.

[Jordan]

Sorry I'm coming in so late. Don had pointed me towards this forum, thinking about this thread, but I didn't notice it earlier.

You can, theoretically, laminate a beam to get a deeper beam from mulitple plies. In practice, it cannot be done safely outside of a factory.  If you bolt or nail, the liklihood of a failure* is extremely high, or the cost will be be so large as to be prohibitive.  To do a bolt-lam would probably require interference-fit holes (a couple thousanths smaller hole than bolt), and more bolts than you would ever care to see. 

If you're curious about the stresses, the key search term is "first moment of area" - that's the hardest part of the analysis, in my opinion.  As you get closer to the center of the beam (top to bottom), the shear stress increases to a maximum. That's why you can build a box beam, but not stack 2 - 2x together. In a box, the fasteners are at the top and bottom, very near the edges of the beam where the shear stress is the lowest.  I know that may sound counter-intuitive that the stress at the top is less than in the middle but it is, in fact, true.  Shear stress is larger at the center, tension and compression are largest at the edges.

The other thing about a bolted, laminated beam is that the deflection at the center may be too great at failure* to accurately account for the deflection fo the beam, which is limited by code (and practicality).

I wish I had a copy of the solutions manual for Beer & Johnson's Mechanics of Materials. Chapter 5 has an excellent discussion of this, and actual problems with nailed up memebrs.


*Failure here is defined based on the 6 NDS equations for yielding failure, not necessarily catastrophic collapse.

[Don P]

Thanks for adding to this Jordan, we're definitely out of my league.
This is an attempt at analyzing the stresses. The numbers above are the shear stress in lbs from the formula w(l/2-x) at each foot along a 20' span and 6000 lb uniform load.

The numbers below would be the shear stresses in psi along the neutral axis if this were a one piece solid sawn beam. The formula there was (3/2)(V/bd)

If I did that right you would then connect each section with enough fasteners to handle the stress in that section? In other words if a 3/4" bolt is good for about 500 lbs in single shear, in the first foot there would need to be 6, appropriately spaced, the next foot would need 6, the next foot 5 and so on, is this the right track? My understanding is this would make a beam strong enough but with up to double deflection. Split rings, which should have much more limited slip and are good for about 2000 lbs each would be a better choice in my mind.

For this beam, to preserve the wood facing and to make a reliable beam I'm leaning towards a flitch plate, would that be a better choice?

[Jordan]

You're on the right track. You can see how many fasteners are involved...and of course with that many fasteners you're reducing the net section of the beam as well as threatening block shear failure of the beam as well.

The deflection is a real guessing game.  It may be as great as 4 times as great as the composite member. If you double the depth of a beam, you increase it's stiffness by (D2/D1)^3, or 2^3=8. So 2 original beams is only 1/4 the stiffness (2 x 1/8) of a full depth beam.

There's actually a simpler method for this case. If you take the tension in the bottom - compression in the top:

15000ft-lb (WL/8) / (Depth = 11/12') = +/-16,400lbs

(16,400-(-16,400)) = 32,800lbs

32,800lbs / 500lbs/bolt = 66 bolts on either side of the centerline.  I think the discrepancy with your 40ish bolts per side is that for 3000# shear, you have an equivalent of 4500# of shear at the interface due to the 3/2 factor in front of V/bd - but don't quote me, I'm just winging it. Anyway - the T-C trick is the one we use for composite steel deck, and the research there has shown that shear stud spacing has little to no effect on the actual performance, so the studs are uniformly spaced.  By the way - I'm calling in sick the day they try to prep this one. ;-)

As a point of reference, flitch plates are roughly 1/2 the effectiveness of a wide flange beam, pound for pound (If I can use 16 or 19lb of steel in a wide flange beam, it would take 30-40lbs of plate steel to provide the same strength/stiffness.

[tsodak]

WOW!!!!




All I can say is that the level of expertise exhibited here is like something I personally have only experianced on such forums. Just goes to show if you ask the right person the right questions you might just get an education as well as an answer.

I am basically at the point where I agree the idea (which made sense in my mind) is out.  I am leaning towards a single massive hardwood beam (in the range of 12x12) or more likely with a manufactured laminate beam. I have done research that I can go to a beam made of 3 laminated 2x12's further laminated into one larger beam.

Two more questions along these lines. One, if I use the laminated 2x12's I would need to box them in to protect them against moisture. I am thinking what i would have to do is cover them with a box of dry cedar and geocell the seems before I tightened the screws on the covering boards.

If I was to cut a beam, I would be cutting it green out of Eastern Cottonwood, populous deltoides. I have got a stack of 24-36 inchers that a neighbor cut down last week, so they are green as grass. I have anchorseal on order to close them up, but as wet as we have been they are not checking yet...(fingers crossed). If I cut a 14x14 beam, I figure it will be roughly three weeks past my 47th birthday (I'm 35 now) before it will be completely dry. Seriously, if I were to use this method, I would lay the top board of the covering on the top flush, then attach the side boards with 1/4" spacers to allow some room for the beam to breath but keep it dry. I guess you would have to leave the bottom open for a year or two to let the moisture out. With the overhangs I would have on my posts if the beam shrunk 1/2 inch on each end in that time it would not cause a problem.

Obviously you are leading the blind down a dark road, but I appreciate any more thoughts from you all.

Tom

[Jordan]

Though I suspect price is the likely issue, why not just get a full glue-laminated timber?  A quick google came up with:

http://www.usglulam.com/products/

They advertise cedar glulams if you're concerned about moisture resistance.


On the practical side, would you consider using knee braces and larger posts?  The normal problem with knees is that they cause havoc in the bending stress of posts, but it may be a way to preserve "most" of the view while shortening your beam span.

[tsodak]

Yes, I think I will consider using knee braces. I have about made my peace wih shortening the spans as well. Hard to give up a vision you know???

Price is a major issue of course, but it is also fairly important to me to do this whole thing with my own materials. NO ONE does that around here, and I think it sends a nice message to use our own local resources instead. Noy one stick is going into this project that would not have imply rotted into the ground or gone into a burn pile otherwise.

With those limitations I think shortening my spans will have to be a main step.  I was hoping for a novel solution from someone out there.... ;D

[Raider Bill]

Though I suspect price is the likely issue, why not just get a full glue-laminated timber? 

In your opinion would these be the most econmical floor joists for a 40x50  ranch over drive out basement?

[LoofO]

Hello all,

I realise this discussion is years old but its one ofe very few mentions of bolt-lam I've managed to come across...

A bit of background... I'm a structural engineer in Cardiff, South Wales (UK) studying part time for a Masters in Structural Engineering (I've been in industry for about 6 years).  I'm hoping to propose a research title for my dissertation relating to 'Bolt-Lam' and variables thereof.  Years ago, my father built a ~12m footbridge using an arched bolt-lam beam.  He tried to progress the theory and design but didn't have an academic engineering background, so now i'm hoping to continue his work.

I would be very grateful indeed if any of you could tell me what you know about the concept, and if you have any design material electronically it would be of significant help to me.

Please do get in touch,
Kind Regards,
Lewis

[beenthere]

Welcome to the Forestry Forum.

Interesting endeavor to continue your father's interests.

What are your limitations on the bolt-lams, such as size of bolts and lams, whether you are considering including ring connectors, what species of lams, what grade of lams, structural sizes included, and the like?
What size of beam will you be testing, type of loading in the test, and what control will you be comparing your tests to in the analysis?
Any glue in the construction of the beam?

[jwilly3879]

A 20' span for the beam is quite long. Here in NY where I live we have an 80#/sf ground snow load which is the major load on a deck (no roof) it may be the same in SD.