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Built-Up Sawn Lumber 2-Span Continuous Beam

Built-Up Sawn Lumber 2-Span Continuous Beam

Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
Background:

I am checking a continuous 2-span built-up sawn lumber beam of a cottage that was constructed within the last year (I had nothing to do with its design or construction). The beam that I am checking is the main floor perimeter beam supporting the main floor, and the wood stud wall above which in turn supoorts a loft and the roof.

Preliminary checking indicates that this beam is over-spanned. The spacing of the supports (number of supports were reduced), and the size of the beam were both increased by the contractor, from the original system designed cottage, but no engineer sized the new setup.

Although certain simpifying shortcuts could be made when designing such a beam, I cannot take such shortcuts when checking because I may put the owner to needless expense strengthening the beam, where perhaps no strengthening is required if more accurate checking methodology is used.

Given:

The 2 spans are about 7 feet and 11 feet, and 2 of the plies of the 4 ply beam are butted at about the 2 foot locatiion from the centre support in one span, and about 3 foot from the centre support in the other span.

Questions:

1. Does Woodworks software account for the location of the joints?

2. When checking manually, how should the butted plies be dealt with? For example, is there a "development length" over which the ply picks up its share of load from the adjacent plies of the 4 ply beam? Are the nails generally adequate to make that tansfer of load?

3. Are "clear" spans rather than centre-to-centre spans entered into Woodworks?

4. Is there any worked example of design or checking of a built-up continuous wood beam with some of the plies butted within the spans?

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

ajk1,

Woodworks does not account for the location of the joints. However, you can input the location of each splice as a point of interest so that Woodworks will provide you with bending, shear, etc... at the specific locations you would like to check.

At the butted ply locations, I only consider the continuous plies, i.e. no moment transfer of discontinuous plies, shear strength of only the continuous plies, etc...

The latest version of woodworks lets you specify which span you have input.

Also, some residential codes have prescriptive methods for splicing beams that is considered to be code compliant. Usually something like, splices of single plies are allowed within 6" either way of the quarter points of the span

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
To Canuck67:

The point of interest feature is good, but I already manually calculated the moment and shear at the splice location, and checked whether the remaining 2 of the 4 plies can resist the moment and shear at the splice location (I find that the remaining 2 plies can resist the moment and shear at the splice location).

But I am still puzzled as to whether that is all that needs to be checked. Is there a "development length" like there is for rebar in concrete? How do the 2 spliced plies pick up the load so that at the maximum mid-span moment location all 4 plies are equally participating in resisting the load?

For example, if the splice point is 3 feet from the centre support of a 2-span beam of 11 foot spans, then the distance from the splice point to the point of maximum span moment is approximately 0.6x11-3 = 3.6 feet. The total factored load on the 11 foot span beam is 19,000 pounds, so each of the 4 plies must support about 4750 pound factored load. How can such a large load be transferred from the adjacent plies with only the nails?

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

ajk1,
You might want to attach a sketch of the way this thing is framed.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
It is a standard built up beam of: 4 vertical plies, each ply a 2"x12" #2 spruce member, 2-span beam loaded on its top, with 2 of the plies spliced at 0.25 of the span from the middle support.

I don't have a scanner here at home. I can try to sketch it up and photograph and then attach the photo.I will send sketch later this morning.

I expect that people who specialize in wood design could answer the question of how to design at, and adjacent to, the splice point, particularly how the load gets transferred from the non-spliced 2x12" to the spliced 2x12's so that at the location of maximum moment all 4 plies carry an equal 25% of the load. Or do they?

Is there a "wood forum" for wood design questions?

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

That would be the NDS forum here. Unfortunately, I think that you're on to something here. I don't see how the butt spiced system works unless the discontinued plies are able to dump their shear reactions locally near the butt joint. That's just statics, right?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

However, most of the guys that will respond in that forum are the same ones you will get here. So it may be fruitless to attempt to repost in that forum.

The "proper" way to design that splice would be to determine the shear and moment transfer required across the splice. Then figure out the number of nails required to take the load from the 2 plys that are terminating into the 2 plys that are continuous across the splice. Then figure out the number of nails to put that same load back into the new 2 plys that are going to continue on from the splice.

Re-reading that it is really awkward sounding and I'm too lazy to go back and re-formulate a response so see my rudimentary flow chart as I see it.

4 plys carrying load ===>> design nails to transfer 1/2 load into continuous 2 plys ===>> 2 plys carry load for short length ===>> design nails to tranfer 1/2 load into new additional 2 plys ===>> 4 plys carry the load

That is obviously not an exact description of how to design it. But that is the load path as I see it.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
TO kootk and Jayrod:

What each of you describe is precisely my thinking...yes statics must be satisfied. It seems to me that the number of nails to transfer a load of this magnitude would far exceed what is normally provided (maybe that is why they say only splice for light loads, but unfortunately they never suggest a load range guide for what would be a light load. I better go thru the calculations and prove it one way or the other.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

Use screws. Not a carpenter's bag of #8 drywall screw, but real wood screws or bolts rather than just nails or staples.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

We've gotta be missing something. I see this system all the time in my area and I don't recall ever seeing a big confluence of fasteners at the ends of the plies. Maybe one just has to accept that the force transfer occurs over a relatively long length. Still, you'd think that there would be rules of some kind.

@Jayrod: nice work with the flow chart. Nailed it!

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

It's not as many nails as you may think. What possibly can your shear load be in a 2 ply beam, a few kips? A 3" common wire nail into the shittiest wood is good for 150 lbs. and the load transfer doesn't need to be instantaneous at the end of the splice, it probably happens over a few feet meaning if you've got 4 rows of fasteners you can transfer 600 lbs per vertical line. Tighten up the spacing in the splice area to 6" O/C and you can produce shear transfer of over a kip per foot of splice.

Maybe I'm over generalizing but like KootK said it's done all the time with no issues.

I'm also of the mind that by the time you provide enough fasteners to transfer the shear then the moment will be taken care of as at 1/4 span it should be near zero.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
Just to refresh your memory, I noted earlier in this string, the beam spans 11 feet max, is 4 plies (not 2), 2 of the 4 plies are spliced at the 3 foot from support location, and the factored load on the 11 foot span beam is 19,000 pounds, so the factored reaction is >> 9500 pounds (due to the continuity, 2 span beam).

Aren't you just transferring the reaction from the partial length ply to the adjacent full length ply, which makes the maximum mid-span moment in the full length ply very nearly equal to what it would be if the partial length ply were not there? That's what it seems to me. I will draw the free body diagram and follow the forces through from ply-to-ply, and span-to-span, and see how it works out. In the end, statics of each ply, looked at as a free-body diagram, must be satisfied.

Yes butted plies are used all the time, but it remains to be seen whether they are equivalent to the sum of all the plies, or only to something less.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

ajk1,
I don't think that I understand how this thing is framed. Do you have (2)-2x12 in the center that are 11' long and (1)-2x12, each side of the center plys that has 2 segments that are 8' & 3' long that are somehow attached to the center plys? If so, then the 2 outer plys won't supply any support to the loads from the floors, roof and walls above.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
Please see attached sketch and comment thereon.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

ajki,
I think that the attachment didn't coperate and go into the "attachment" mode

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
I tried it again. Did it upload this time?

One way to settle the issue, is to run the 4 plies on a grid analysis program, with the load applied along the top of each ply, and have a fictitious short member spanning across the end of the spliced plies and connected to the plies each side (simulating the nails). Then see if the mid-span moment in each ply is the same as if there were only 2 plies or the same as if there were 4 plies.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

For the 7' span having a splice at 2' in from the end I get the shear at the splice as ~1600# if the beam is uniformly loaded (1055 plf). For the 11' span it is ~2700#. Those are total shears and so you can reasonably assume each ply carries 1/4 of that which mean you need to design nails for 400lbs at each location for the 7' span and 700 lbs for the 11' span.

So from each continuous member into each of the two discontinuous members you need like 5 nails. Put 4 rows of nails at 6" and you've got 8 nails in a 1' length like I mentioned earlier that's over a kip per foot of shear transfer.

Am I missing something?

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

While I don't have an answer yet, I have some more observations to share:

1) When I opened AJK's sketch, my instantaneous gut reaction was "that's a two ply, continuous beam".

2) This will sound impossibly lame but my next step was to ask my wife. She's also a structural engineer and something of heavy timber specialist. Her gut reaction was the same as mine. She said that she encounters this a lot in older buildings but never does it for new construction. My wife rarely deals with part 9 stuff though and that's where I've seen most of the newer, real world examples that I mentioned above.

3) Assuming that the maximum shears occur near the butt joints, I think that we can all agree that the beam is a two ply member from a shear resistance standpoint. Of course, it's unlikely that shear would govern.

4) If the four plies all experience identical vertical deflections along the length of the beam, then they would all have the same curvatures and therefore the same moments and shears along the lengths of the beam. This depends on the ability of the nails to do their job convincingly. And, clearly, this compatibility of curvature would be somewhat out of whack locally near the but splice.

5) I feel that the stiffness of any moment connections near the butt splices is just as important as the strength of those moment connections when it comes to maintaining compatible curvatures amongst the four plies. When I was a kid, I tried to construct a portal frame fort from three 2x8's nailed together at the lapped corner joints. It sucked. Based on that limited anecdotal evidence, and the fact that you have to make such a connection twice as you cross each butt joint, I question whether or not moment transfer across the butt joints should be relied upon for enforcing curvature compatibility, and true load sharing, amongst the four plies.

6) Based on the above, I think that the composite beam is clearly better than an analysis of two plies would suggest but also definitely worse than an analysis of four continuous plies would suggest. For new construction, I would only count on two plies. I'd only tinker with four ply voodoo if it was an existing condition and I was a bit desperate. From what I've read, that's exactly where you're at AJK.

7) As a way to evaluate an "in between" solution, perhaps one could abandon the notion of moment continuity across the butt splice while still satisfying shear transfer requirements. You wouldn't have perfect 1/4 load sharing any longer of course. However, the spliced plies could still make a meaningful contribution at the maximum moment locations. Again, I would never go to all this trouble for new construction.



The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
To Kootk and your wife: You are a power couple. I had not thought about this from the point of view of curvature but that is interesting and logical. I am not sure that the nailing can cause all the plies to have the same curvature though. I think we are both at the conclusion that we would design this as a 2-ply beam if it were a new design, and that would be a little conservative. I am not that "desperate" about this since it is not my design and I had nothing to do with the construction, and it appears that the contractor just guessed at things. I think I will most probably recommend additional piers where it does not work as a 2-ply beam. I will further study what you have sent...looks like a lot of very good thoughts there. Thanks for taking the time to think it through and write it all out. Much appreciated.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

ajk1,
1. I assume that 2 of the 4 plys don't have the required capacity in the 11' span and you did not design the existing beam.
2. You noted in your first post that this beam is supporting a floor, a loft, the roof and two walls, so it is a key component of the support for the house.
3. As KootK noted it is damn tuff to design a moment connection is a wood beam.
4. I would throw in the towel and design a new beam that you know will support the design loads. You will sleep better at night time.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
That would be very expensive...have to temporarily support the cottage while pulling out the old beam. I would expose myself to liability because the contractor would sue me for not designing an economical fix that he would have to pay for. Then I would not sleep at night. If the cottage were not all built and completed, then I would agree with you. As a lawyer pointed out to me 30 years ago, when I first prepared to act as an expert witness, repair/remedial measures are not the same as new design. There is a professional and legal obligation to minimize the remedial costs, consistent of course with providing a safe design that can be shown to satisfy the intent of the Code. But thank you for urging me to sketch it up so everyone had a clearer understanding of the set-up. Much appreciated.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

You're quite welcome AJK. It's an interesting question and I do owe you a few favors for your past efforts on my threads.

My interests are 80% technical / 20% project management. My wife's are the reverse. You can imagine where the bulk of the "power" resides in this couple.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

I think you are all being overly conservative. Don't know which code you are using, but the 2006 Alberta Building Code Part 9, Article 9.23.9.1.3) states:

Quote (ABC 2006)

Where a beam is continuous over more than one span, individual members are permitted to be butted together to form a joint at or within 150 mm of the end quarter points of the clear spans, provided the quarter points are not those closest to the ends of the beam.

I do not have a later issue of the ABC but this requirement has been unchanged for as long as I can remember. The Alberta Building Code, so far as I am aware, is consistent with the National Building Code of Canada but I cannot vouch for codes outside Canada.

BA

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

Nice find BA. I'll still need to understand the mechanism before I dive in however. I feel like a bad Canadian for not knowing about this provision. I'm pretty sure that I did this a few times as an EIT without even thinking about it.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

Technically, you would need two additional sets of bolts on the other sides of the butt joints to get the shear back into those outer plies as you move towards the supports. Also, to the extent that there is moment at the butt splices, I still question the stiffness of a nailed/bolted moment connection in wood.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

You can add more bolts if you like, but I don't think it is necessary unless governed by shear. Plies 2 and 4 act as 7'-6" long simple spans and need bolts to transfer a reaction of 7.5w/4 to Plies 1 and 3 where w is the uniform load per foot on the 4 ply beam.

The moment at the splice point is close to zero because it is close to an inflection point, so the connection is not considered to be transferring any moment.

BA

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

We must be seeing this in quite different light BA. Are you considering the beam as two simple spans meeting at a common support? With only the two bolts that you've shown, you'd only have continuity in two of the plies over the center support, where one would expect the moment demand to be highest?

Also, was the ABC quote from Part 9?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

To add to KootK’s thoughts and observations list, 29AUG, 12:35....

2 cont.) We all know that when all else fails, ask the boss. So, that’s not lame, that’s just self preservation, and getting it straight from the horse’s mouth. Remember, when the boss is happy and involved, everyone else is happy too.

3,4,6 & 7) I essentially agree.

5 cont.) Your portal frame corner joint may have turner out to be a real educator, but it was probably also the worst (most difficult) possible case of trying to develop a moment in wood. You can induce quite a moment in that corner joint, with little effort, and you only have about a 6"x6" face area into which to place the locking nails or hardware; small moment lever arms btwn. fasteners and can’t get a enough nails in that area, edge dist. and splitting, etc. I think our problem here is slightly different since the individual members are aligned, and we have a much longer distance over which to place nails and start to develop a moment transfer/shear transfer. And still, the moment transfer is not exactly the same, and that’s where your ‘do they deflect together, if so they all carry load approx. equally’ comes into play. We aren’t really transferring moment at the butt splice, we are transferring load or shear all along the members to make them act in unison, and need to add some extra fasteners around the butt joints to try to control their differential movement at those joints.

At the neg. moment, you are not really providing moment continuity at the butt joint, and the moment is low there in any case. You are transferring load or shear through the joint, out at 2' or 3' (or less lever arm as you move to the post) and this causes the spliced members to participate in the neg. moment cap’y over the post (i.e. taking their 85 or 90% of one quarter share). And, there will likely be some curvature/deflection (irregularity/discontinuity) in the immediate area of the butt joints. Thus, my guess is the max. neg. moment is probably carried by about 3 or 3.5 - 2x’s, not all 4. I would pretty much assume all 4 -2x’s acting at mid-span, because you have a longer dist. to develop their continuity from both directions.

8) The factors of safety and material strength reductions are really quite high in the wood codes because of the non-homogeneous nature of the material and the possibility of sizeable discrete defects. And, you have the normal plywd. or LVL redistribution of weak spots which lead to a stronger member. So, in part, they are probably working into our FoS.

9) Loading on top vs. hung loading from one side... In the first case we have a floor framing system and then a sheathed stud wall which loads the beam from the top, probably all four members fairly uniformly. Then, the stud wall acts kinda like a deep beam, and I’ve actually seen them span long distances when the beam or found. wall failed. I would watch large jamb loads which tend to concentrate loads and may change max. moments or shears. In the second case, your fastening system is having to transfer the whole load (lbs./lin.ft.) through the first beam element/ply into the other three to make them act in unison. Two pretty different connection problems.

10) BA’s ABC find should sound about right to all of us, an approx. point of contraflexure. The moments will be fairly low in that span length region, and the shear stress will only be about half of the max. shear at the reaction points, and as mentioned above shear usually is not critical anyway.

I would pay some attention to the potential for unbalanced loading and see how that might move the shear diagrams and moment diagrams around at the butt splice points. In this regard it would be helpful if Ajk1 would provide those load, shear and moment diags. since he’s been doing the calcs. Then also the member calcs., stresses, etc., at mid-spans, over the middle post and the max. shears at reactions. I’m not going to do them. What does the bending stress vs. allowables look like at mid-span Bm. #2 and at the negative moment? What about horiz. shear at the worst reactions? And, what are the moments and shears at the butt splices, for comparison?

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

Quote (KootK)

We must be seeing this in quite different light BA. Are you considering the beam as two simple spans meeting at a common support? With only the two bolts that you've shown, you'd only have continuity in two of the plies over the center support, where one would expect the moment demand to be highest?

Also, was the ABC quote from Part 9?

I agree with what dhengr stated above. The beam is semi continuous. There may be an effective section of 3 or 3.5 plies resisting the negative moment at the central support.

If there were no nails between plies, each ply would deflect separately. Ply 2 and 4 would act like a simple span of about 8'. Their left reaction would produce a concentrated load on plies 1 and 3 which would deflect relative to plies 2A and 4A. So the 3' long cantilever of plies 2A and 4A would tend to hog the uniform load from plies 1 and 3, partially compensating for the concentrated load mentioned above.

There probably are nails between plies, so the above is a worst case scenario. A regular spacing of nails between plies will tend to improve the performance. Butt joints are not and should not be considered capable of carrying moment.

Finally, the ABC quote was indeed from Part 9.

BA

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

All summed up, I would say that we can all agree that the fastening of the plies is quite important to the assumption of load sharing. And to bring it back to AJK's original point, force transfer at the butt joint seems as though it would require consideration of localized nailing to transfer at least shear, if not moment. That's not what I see in the field however. And I'm not aware of any code guidance for ply fastening other than generic "two rows at __ o/c" kind of stuff. If designers are supposed to be giving it special attention, I don't see that either. I still see a problem here.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

I agree that fastening of the plies is important, particularly at the end of ply 2 and 4.

I just had a quick look at Wood Design Manual 2001 in the "Built-Up Beam Selection Tables". For No.1/No.2 SPF, a 4 ply 38x286 (2"x12") beam has a factored resistance of Mr = 24.2kn-m (17,860'#) and Vr = 28.7kN (6,460#). That does not seem adequate for the factored loads given, even if all plies were fully continuous over the two spans.

BA

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
If all 4 plies are equally effective, and the beam +ve and -ve moments are determined based on continuity, and accounting for the preservative treatment incisions (Kt=0.75), Woodworks says that the beam is over-stressed in flexure by 33%. Woodworks puts pattern loading on. The maximum positive and negative moments are very nearly equal.

There is significant moment at the splice point, but this can be taken by 2 of the 4 plies, without over-stress. However, the moment that should be used for this check should perhaps be a little further out than the actual splice point.

I agree with much of what you both say. I have the Woodwporks output at the office but I will try to retrieve it here at home and send it soon.

I do not find in NBC 2010, the clause BAretired notes is in 9.23.9.1.3 of the Alberta Code. What is the title associated with clause 9.23.9 in the Alberta Code? I do recall reading something like that, somewhere, but as I live in Ontario, it would not have been in the Alberta Code. I will continue searching for it.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

You got repetitive member in there too AJK? That might help make up a little ground.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
What do you mean by "repetitive member"?

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

If you have three or more of the same members sharing load, and they're not spaced too far apart, you get a nice little bump on you allowable stress. It applies to multi-ply beams. If your using wood works, you're probably taking advantage of this automatically.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
yes i believe so.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

ajk1,
Is there some reason that haven't mentioned adding a new footing and post either at mid span or under the spliced plys? This may reduce the stress enough so that two plys will definately handle the loads.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
Yes an extra support is precisely what I have been considering. It should not cost that much, but the cottage is reachable only by boat, so it is a bit of a bother to bring all the materials (for concrete footing, block piers, etc.) but still the most cost effective solution. I just wanted to be sure that I was not doing something unnecessary.

I note that the Span Book permits a live load reduction factor of 0.8 for "lintels" supporting 2 floors and a roof. Why does it not say anything about this for a beam supporting 2 floors and a roof? Although this will not get it out of the over-stress condition, I was just wondering why this is limited to lintels.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

Hold the phone... I see cantilevers that have mucked up the whole "insignificant moments at the quarter points" business.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

Seems to me the cantilever on the right can do nothing but help the situation for the 11' span. The cantilever on the left is of little consequence.

BA

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

For a gazillion posts here, we've been hanging our hats on the butt joint being fine because it's a point of inflection. Now, as a result of the left cantilever, there is no point of inflection. Rather, the butt joint see a max moment upwards of 2/3 of the maximum negative moment at the support. In the context of this discussion, that seems pretty consequential to me.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

Ajk1:
When you get done with this beam problem, take a look at the lateral loads on a whole bldg., going through the first fl. diaphragm, then through two or three standing beam lines sitting on top of some posts sticking up out of the ground. How many posts, 6, 9, 12, and how deep and on what footings. What are these posts and the post/beam connections good for laterally? A 6 or 8" post cantilevering a couple feet above grade and buried a few feet in a loose filled hole is not worth much laterally.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
yes dhengr, that is very perceptive and the next item on my "to do" lists of things to check. The lateral load issue is more severe than you suspect, because the whole cottage is several feet (7?) up in the air! I was going to put the lateral load issue as a new string, but you beat me to it.

And some of the posts are eccentric on the piers, the piers have no mortar in the joints, the footings may be undersized, and on and on it goes with odd-ball things.

The loads on the cantilevers are quite small, so I don't see that they change the basis of the fundamental question. If the cantilevers were not there, the question would be the same, namely can we assume all 4 plies resist the load equally, as implied by the Alberta Building Code?

When I get into the office Tuesday, I will see if the 2010 NBC Part 9 is like the Alberta Building Code. I will also have to have another look at CSA O86. I should also check ASCE although our copy of it is quite old.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
ok

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

A lot of cottages in my area are built on hilly sites and have downhill piles/piers that project fairly far above grade. It's quite common to see the piers cross-braced from grade to the tops of the piers.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
How do they cross brace concrete masonry piers?

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
How do thye cross brace masonry piers?

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
The footings are below grade and at significantly different elevations. The wood would be in contact with the soil. Connection of the wood to the concrete masonry pier may be problematic. I have done that sort of supplementary steel support, to concrete beams in parking structures, but I am not so sure about it for this cottage; I will give it further thought. Thanks.

I wonder about placing carbon fibre reinforcement each side of the piers to give it some tensile strength.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

Missing from this discussion is how the floor joists connect to the built-up beam.

If they bear on top of the beam, then (for the most part) all plies should have the same deflection profile.

However, if the floor joists connect to one side of the beam, then the distribution of the load to each of the 4 plies becomes rather gnarly, unless the joist hangers are through-bolted to the built-up beam.

KootK's point #6 is true in either case.

Obviously, the connections from ply-to-ply will influence the load-sharing capability of the 4 plies.

If I were to try to do a comprehensive analysis of the condition, I would first examine a 2-span, 2-ply condition cantilevered over the center post, presuming that where the 2-ply beams overlap the load is shared equally. If that proves to be satisfactory I would stop there.

Trying to take advantage of the (2) 5' long plies on the left side and the (2) 8' long plies on the right side really seems like a futile effort. They offer little additional shear & moment capacity unless one gets into a rather complicated analysis of how the plies are fastened together.

Ralph
Structures Consulting
Northeast USA

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

@AJK: they don't brace masonry piers. For some reason, I was thinking steel supports. My bad; false hope.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

PWF material is frequently placed below grade in preserved wood foundations. A knee brace at each end of the 11' span down to the footing would reduce the beam span and provide some lateral resistance. The connection could be to the concrete footing, not the masonry pier.

BA

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
Yes I suppose although we always avoided wood below grade in our projects, even though it is permitted with the right treatment. It is something to keep in mind.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

After viewing the photo in thread507-370835: Concrete masonry piers with no mortar in bed joints, there seem to be more serious concerns with respect to this cottage than the strength of the wood beam.

From my own experience with contractors who produce work of this caliber, I believe that you will have a great deal of difficulty in getting him to comply with measures which you deem necessary for a safe structure. Nobody could blame you for opting out now. If you decide to continue, be prepared to be frustrated throughout the remedial work. Good luck.

BA

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
The owner is withholding a large chunk of money so the contractor appears at the moment to be cooperating.
I have told the owner that we don't want to do it.
The contractor says he has built cottages this way for 30 years, including elevated cottage and they are standing up.

To try to resolve the effect of the butt splice of 2 of the 4 plies, I ran the built-up 4 ply beam on Woodworks with the 2 shorter discontinuous plies being supported by the 2 adjacent continuous plies. i.e. at their end at 0.28 of the span from the support, they apply a load on the 2 adjacent continuous plies. The results were as follows:

For the original run of 4 un-spliced plies (i.e. each taking 25% of the load and fully continuous:

Vf / Vr = 1.21; Mf / Mr = 1.35

With the shorter plies dumping their load on to the adjacent continuous plies:

Vf / Vr = 2.06; Mf / Mr = 1.86

From which I conclude that for moment should consider about 70% of the Mr of all 4 polies, and for shear should consider about 60% of the Vr of all 4 plies. This was for the butt point at 0.28 of the span.

I know this is very rough, but better than nothing. For new construction I would consider only 50% effective, but for checking an already constructed building, the above percentages may perhaps be acceptable, until someone shows otherwise. I have sent an email to the Canadian Wood Council to ask them this question about how to deal with splices.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
The Canadian Wood Council has responded to me and they say that it is standard practice to consider all 4 plies equally effective, provided OBC Clause 9.23.8.3 is complied with, inlcuding the somewhat more rigorous nailing.

I see OBC Clause 9.23.8.3 is similar to the Alberta Code Clause that BARetired referred me to earlier. So looks like BARetired (and perhaps others) was right!

(I remain a bit dubious of the prudence of this practice though, partly because the Canadian Wood Design Manual says that "Built-up beams are often used where the loading is light and dimension lumber is used for secondary framing, thus simplifying the type of materials required for the structure").

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

But even if you consider all four plies effective, the beam is still overstressed, so remedial measures are required. If you can combine the remedial measures for the beam with measures required to provide overall lateral stability, that would kill two birds with one stone.

BA

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
Yes BARetired, precisely! That is what I would like to do if possible. So next step is for me to calculate the wind load.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

Much of the bulk material could be taken out to the site over the ice, during the winter, thus less need for boat traffic. I like the idea of the continuous strip footing around the exterior, with some conc. blk. stem wall up out of the ground, and then a stud wall up under the beams. I would set this wall so that the ext. sheathing went up onto the ext. face of the beams, and rim joists at the main fl. diaphragm. I would also work on details which eliminated the existing blk. piers and wood posts under the exterior beams. The beams will act as the cont. top pl. type tie around the bldg. if tied together at the corners. I would use a single top pl. on the stud wall, under the beams. Lay the studs out so portions of the wall can be built to within a few feet of each pier. Then remove the posts, piers and their footings, and infill the cont. strip ftg., stem wall, and stud wall to complete the exterior basement walls. You may still need to do some remedial work on interior beams and piers, you haven’t revealed anything about them. This gets you up to the main fl. diaphragm with a meaningful lateral support system. And, it still sounds like there are plenty of problems above that level.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
Thanks dhengr. The height of an un-reinforced block wall, with no lateral support along its top, is limited to 3 times its thickness. So 8" block can go up only 24" unless there is lateral support at its top. Also, frost protection there is about 5'-6 or more, according to the frost depth maps, so would involve some excavation where it is not on rock. But may have to do that if can't think of anything else.

Most of the cottage was built in the summer of 2013, so they seem to be able to get materials to it by boat.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

I just took a quick look at the WoodWorks output provided, but admittedly have skipped many of the posts in between so apologies if I repeat something already commented.

It appears that the 2x12's are pressure treated, based upon the Kt values shown. As per the CWC Wood Design Manual, this strength reduction is typically not required for most types of current preservative treatments which have little to no affect on strength, unless the 2x12's were incised. Were the 2x12's incised? You could pick up 25% capacity if not incised.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
Yes they were incised. A eek ago we had the wood inspected for the incision marks and they were definitely there. My understanding is that virtually all pressure treated wood with preservative is incised in Ontario.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)

I was puzzled as to why I could not get the same answers as in th Span Book. Below is the response that I got from the Canadian Wood Council. Any comment? The whole thing seems very confusing to me. In checking an exisiting built up beam, should I use the additional 1.2 multiplier (which is in addition to the 1.1 multiplier) for resisting moment?


1) The Wood Design Manual (WDM) follows the CSA O86-09, and uses the system factor for built-up beams of 1.1 (Table 5.4.4, Case 1), while the Span Book follows Part 9 of the NBC, which allows for an increased system factor of 1.2 as well as an additional construction factor of 1.1 (See page 28 of the Span Book). The increased system factor and additional construction factor are not mentioned in Part 9, but is allowed based on good practice.

Example: Span governed by bending, width = 3.6 m, 3 ply 38x286, S-P-F No. 1 & 2, two floors in a house, the Span Book says that the span is 2.79 m

Factored wf = 3.6 x (2x0.5x1.25 + 3.3x1.5) = 22.3 kN/m
Mr = 18.2 kNm x 1.1 (construction factor) = 21.78 kNm
wL2 / 8 = 22.3 x 2.792 / 8 = 21.7 kN.m, therefore Mr = Mf

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

I don't understand how the O86 would only allow a 10% increase but the NBC allows a 32% increase but doesn't directly reference it anywhere.

And miraculously your example fails as per the CSA O86 by 21% (according to woodworks)

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
Yes thank you Jayrod, I have precisely the same discomfort as you.

I think I will draft a letter to NBC, Ontario Building Code (OBC) and CSA and the Wood Council about this. Seems like lack of coordination between the groups, as well as lack of clarity.

Probably too late to do anything about it now for the upcoming 2015 editions, except perhaps the Wood Design Manual and Span Book and the OBC. Maybe some others should also send letters if they are in agreement that there is a problem

I wonder if the U.S. has something similar to the NBC Part 9 for small buildings and to the Span Book and what they say. Would be interesting.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

Part 9 has always been less restrictive than Part 4 of the NBC. Last time I wrote to NBC/CSA pointing out contradictions, I was told that Part 9 is based on traditional usage and if I was aware of a specific situation where the use of Part 9 resulted in failure or unacceptable performance, to let them know. I had no examples to offer so I let the issue die.

Good luck with your letter.

BA

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

When I interned with a very old and well established Structural Engineering firm the differences between Part 9 and Part 4 were one of my first lessons.

If you think the spans look dodgy, try doing any of the most basic vibration checks for the tables in Part 9.

If you think that's bad enough, try to make the Part 9 handrails work.

If you don't like any of that, then don't look at the foundation provisions of Part 9. They're practically the root-mean-square of what you'd really need.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

So because contractors have been using undersized beams for the last bunch of years (traditional usage) they are allowed to deviate from the design standards? That is a cop-out answer if you ask me.

Remember that for span tables to be used the member must meet the specific criteria laid out in part 9 including uniform loading. Once the loading is non-uniform or a point load is to be supported by the span (which is becoming much more common as the desire for open concept floor areas grows) then you cannot use the span tables and it kicks you back to using the O86.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

I agree Jayrod, wholeheartedly, but that it not what really happens... Even worse you should take a look at some of the TACBOC details of what is permitted without an Engineer's involvement in the Toronto area. Shocking.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

TACBOC = Toronto Area Chief Building Officials Committee

Does Toronto have building standards which differ from the Ontario Building Code?

BA

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
I am not quite so much against the leniencies of Part 9, as I am against the lack of clarity and explanation, and lack of some commentary to bring these differences to the user's attention. This would apply particularly to the Wood Design Manual...and to Woodworks program...or are they intended to be used only with Part 4?

Jayrod12 makes some particularly good points; along the same lines, I have a perimeter built-up beam, but I suppose I can use neither the Tables in the Span Book because they do not include perimeter built up beams, nor can I calculate it using the 1.2 x 1.1 factor for bending in the Span Book ...or is it ok to calculate it using these factors even though there are no tables for perimeter built-up beams?

As for clarity, the NBC and provincial codes should be make it clear that when a multi-span beam has butt joint in the span for some of the plies, that its strength can be calculated based on all plies being equally loaded (if loaded from the top) and based on moments derived from full continuity of the member, if the Code requirements for splice locations , nailing and the like are followed and the strength is checked at the splice point based on only the non-butted plies. Currently it leaves it to your imagination as to whether all plies are equally effective, etc.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

They used to, but have (officially) adopted the OBC because of a provincial law. I know a few of the Building Officials around here (Ottawa) and one is a close personal friend from way back. He's flat out told me that Toronto has a reputation for behaving as if they still had their own code, frequently permit works based on the standards they applied before the OBC became the whole-province mandatory code, and permitted construction based on "custom and practice" reflected in the TACBOC standard details books that Ottawa wouldn't even touch with a ten foot pole.

Some of this may very well be "little City" people being resentful of "big City" status within committees, etc, but I trust the source of this info implicitly. That and I've seen some real disasters (no pun intended) with permits and without a prayer in the Toronto area.

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

(OP)
I live in Toronto and am not aware of what you say. I would be very surprised if that were true, at least in terms of structure. I have no knowledge in non-structural matters. Anyway, the structure to which my questions are addressed is near Bracebridge, nowhere near Toronto.

My experience is that as far as structure is concerned, building departments are in general not very proactive. They are most concerned about collecting the permit fee, and that the submitted drawings not be in colour and the like.

By the way, a house under renovation in Toronto, about 2 kilometres from where I live, collapsed yesterday, reportedly while the foundations were being underpinned,and one worker was killed. This is the 2nd time that I recall a house collapsing while being underpinned in Toronto in the last two years. You would think that the building department would be more vigilant on house underpinning, but I wonder if they had a building permit...

RE: Built-Up Sawn Lumber 2-Span Continuous Beam

Look up the TACBOC standard details, then tell me how surprised you are.

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