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?
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
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
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
You might want to attach a sketch of the way this thing is framed.
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
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
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
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
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
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
@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
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
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
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
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
I think that the attachment didn't coperate and go into the "attachment" mode
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
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
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
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
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
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
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
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 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
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
BA
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
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
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
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
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
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
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 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
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
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
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
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
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
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
I don't have the latest code, but I am attaching an excerpt from ABC 2006, Part 9 with the paragraph I quoted earlier.
An extra support suggested by OldPaperMaker might be the easiest solution.
BA
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
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
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
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
BA
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
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
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
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
BA
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
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
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
BA
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
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
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
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
BA
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
RE: Built-Up Sawn Lumber 2-Span Continuous 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
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
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
BA
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
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
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
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
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
And miraculously your example fails as per the CSA O86 by 21% (according to woodworks)
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
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
Good luck with your letter.
BA
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
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
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
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
Does Toronto have building standards which differ from the Ontario Building Code?
BA
RE: Built-Up Sawn Lumber 2-Span Continuous Beam
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
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
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