double wood beam - shear control?
double wood beam - shear control?
(OP)
Hi, maybe that is a silly question but its not my area...
Lets say we have a roof wood beam (element 1), but its not sufficient on its own (shear forces too big) so we add another wood beam (same material) on top of it (element 2).
Can I consider this now as one element when making control for shear forces? Does there have to be connection?
I know it has to be connected when dealing with bending (shear flow between elements so we need bolts or other connectors) but what if I know that element 1 is Ok as far as bending goes on its own? Because of that i really want to avoid calculating shear flow and bolts and calculating effective moment of inertia Ieff - because of a slipping between elements, etc.(according to eurocode 5).

BTW i have to connect elements somehow so bolts should be OK?

Lets say we have a roof wood beam (element 1), but its not sufficient on its own (shear forces too big) so we add another wood beam (same material) on top of it (element 2).
Can I consider this now as one element when making control for shear forces? Does there have to be connection?
I know it has to be connected when dealing with bending (shear flow between elements so we need bolts or other connectors) but what if I know that element 1 is Ok as far as bending goes on its own? Because of that i really want to avoid calculating shear flow and bolts and calculating effective moment of inertia Ieff - because of a slipping between elements, etc.(according to eurocode 5).

BTW i have to connect elements somehow so bolts should be OK?







RE: double wood beam - shear control?
RE: double wood beam - shear control?
But im wondering about vertical shear forces that act on beams around supports.
BTW there is no loading on element 1. the load will be applied after adding element 2.
Do you think connecting elements with vertical bolts alone is OK?
RE: double wood beam - shear control?
I would still stitch them together nominally though considering seismic forces.
Mike McCann, PE, SE (WA)
RE: double wood beam - shear control?
If bolts can develop zero horizontal shear on the interface, Member 2 will carry 1/8 of the load that Member 1 carries, so the shear stress in Member 1 when acting alone will be reduced by a factor of approximately 8/9.
BA
RE: double wood beam - shear control?
BAretired, how did you get this values: "If bolts can develop zero horizontal shear on the interface, Member 2 will carry 1/8 of the load that Member 1 carries, so the shear stress in Member 1 when acting alone will be reduced by a factor of approximately 8/9."
RE: double wood beam - shear control?
14^3 = 2744
28^3 = 21952
Therefore 14 cm beam takes 2744/(2744 + 21952) = 1/9 of total load.
RE: double wood beam - shear control?
As pointed out, #2 may do little, or, depending on how it's loaded and supported, it may do nothing.
It's probably not too difficult to get them to act compositely, and the stiffness in triples in comparison to the non-composite version.
RE: double wood beam - shear control?
RE: double wood beam - shear control?
Maximum vertical shear is 40 kN at supports.
Timber is C16.
Lenght of the beam is cca 500 cm.
RE: double wood beam - shear control?
It would probably be much easier to bolt beams on the side.
I'm personally suspect of the composite beam condition. Some degree of bolt slip would be expected as the bolt holes would be oversized slightly. I'm not sure if that slip would be enough to interfer with the composite action. My preference would be to bolt new beams to the side of this one or if it is new construction to replace with a new full depth beam.
RE: double wood beam - shear control?
In terms of mechanics of materials, you're good to go. The upper member will even take a bit more load than the stiffness ratio predicts because the upper member will be bent to a tighter curvature than than the lower member.
My concern is the details of how the shear in the upper member will make it to bearing. It seems to me that shear would have to pass through the lower member. If that takes place within 28 cm of the support, it's probably fine. If not, then you may not have actually reduced the maximum shear in the lower member as much as you think, or even at all. So I guess it comes down this: how confident are you that the upper member dumps it's shear near the low beam support? For me, the answer would be not very
I'll concede that, because wood shear failure is dominated by horizontal shear concerns, this is not as serious of a thing as it might be with, say, a concrete beam.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: double wood beam - shear control?
If the span is 5 m the maximum moment is 50 kN-m
I'm not familiar with the properties of C16 Timber but I suspect that the 200 x 280 beam is overstressed in both bending and shear. Also note that the maximum shear stress in a rectangular timber beam is 3V/2A.
M = 50 kN-m. S = 200(280)2/6 = 2.61e6 mm3 Fb = M/S = 19.2 N/mm2
V = 40 kN. A = 200*280 = 56,000 mm2 v = 3V/2A = 40000/56000*3/2 = 1.07 N/mm2
These values appear too high for a timber beam.
I can't say I am enthusiastic about using bolts in the proposed manner. Perhaps another solution should be considered.
BA
RE: double wood beam - shear control?
Problem is that contractors already has element 1: 20/28 cm but its not suffiecient material. They want to keep this element and add another on top of it (element 2).
The actual model is like this:
i was thinking something like this:
RE: double wood beam - shear control?
RE: double wood beam - shear control?
RE: double wood beam - shear control?
BA
RE: double wood beam - shear control?
To account for the bolt slip and likelihood that not all of the bolts will be engaged at the same time for the composite action (due to bolt slip in the wood members), I would personally 2x-3x the # of bolts required. I suspect you'll find the bolt requirement is not very large for the composite action so I don't think this should be hugely problematic.
RE: double wood beam - shear control?
RE: double wood beam - shear control?
RE: double wood beam - shear control?
It never ceases to amaze me how you OP’ers. dribble out important structural info. during the first 19 or 20 posts (responders guessing at what you are really doing), while we try to drag the info. out of you. Maybe your ‘Agreement about double wood beam and bolts only,’ should have included the ‘silly’ proviso that it, and the who structure work, actually work structurally; a small detail that the contractor might miss in asking you to save his a$$. You might also consider involving a local Structural Engineer, if this is ‘not your area.’ What happens in the other elevation or in plan, is still an important structural consideration, and is also still a mystery.
I would start with well seasoned timbers (kiln dried) to minimize shrinkage. I would use some of the newer structural screws, one brand or another, instead of through bolts and structural glue in the joint to minimize the joint slippage and max. the shear transfer. You could put a couple split rings in the joints at each end of each beam to improve shear transfer and composite action. You might wish that you didn’t have hinges at the joist/beam bearing plane, and at the beam/column bearing elevation, and you should do something about this. Given what we’ve heard so far, your many 45̊ kickers will probably be very difficult to do well with wood. You might consider pre-fabed steel kickers that through bolt at the column and screw up into the beam. These would be 24-36" long, 6" wide by .25" thk., with a welded 2x2 HSS diags., black wrought iron in appearance. These can be made with some degree of precision and the connections can be made with some degree of confidence. Note also, that with the use of the kickers, you have to hold the beam ends down to the top of the columns for things to work properly. Thus, I would add some “T” straps btwn. the two beams and each column; this holds the beams together lengthwise and holds them down on the column. Finally, I would put the added beam member on the underside of the larger beam member, and make it only 5m, less a full column width long, to fit btwn. the columns. It would bear on the kickers, which are pressed up tightly to its underside before they are attached to the column by through bolting. I would set the beams on a 1/16" shim atop the column (some engineering judgement and thought is needed here). Once the kickers are attached, I’d pull the shims, press the upper beam down onto the column and fix the “T” straps in place.
While this may seem a little fussy, remember, you are working to save the contractor’s ‘bull,’ and he should now make an effort to make the whole thing work right now.
RE: double wood beam - shear control?
It appears to me that the contractor may have to accept a compromise.
BA
RE: double wood beam - shear control?
I haven’t run any numbers, as you were so kind to do for us, for lack of OP’er. basic structural info., so I’m not absolutely sure how it would shake out either. I was not intending that the end shear pls. would take all of the horiz. shear. They would just take the higher shear flow at the ends, where the alternative would be 10-12? structural screws in 16-24". They would also provide some protection against the horiz. shear/notch affect/reentrant notch at the end bearings. I would still space struct. screws over the length of the beams to take most of the horiz. shear in a more distributed fashion along the beam, and to clamp the two parts together. I suspect the struct. glue/adhesive would do the trick over the middle half of the beam, as long as it is protected from environmental conditions, and the codes would allow us to start using struct. adhesives. I like the struct. screws from below because they are protected from water standing atop the beam. We don’t know what the roof protection is, do we? My intention for the 1/16" shim atop the column, then its removal and pressing the beam down, is to really seat the lower beam bearing on the horiz. leg of the kicker to assure some distrib. of the vert. reactions, btwn. the kicker and the column top.
RE: double wood beam - shear control?
if this is right, it means that bolts dont contribute much!
gama = 1,0 means complete 100% connection between elements
gama = 0,0 means there is no connection
for bolts M14 (8.8) at 50 cm spacing I got gama = 0,023 which is almost zero, but enough for controls to be OK...
Bolts alone are pretty ineffective because they dont produce enough resistance for shear flow development... if i choose smaller spacings its still pretty ineffective.
RE: double wood beam - shear control?
You started out with 5 m simply supported beams. Your shear force diagram seems to be taking into account some form of continuity, but it is not clear which spans are loaded and which are not. In order to analyze the structure, you would need to know the stiffness of the columns. I don't believe your shear diagram is accurate.
It might be prudent to retain a structural engineer to analyze the structure properly and to make recommendations to the contractor.
BA
RE: double wood beam - shear control?
I haven't used structural screws to any extent but would have to penetrate 140 mm (5.5") before becoming effective so the screws would probably by nearly 12" long. Would that be efficient? Would lag screws be more economical?
I like the idea of using structural adhesive, but I'm not sure it would be effective in an exterior exposure. My knowledge of adhesives is not current.
As for the kickers, I don't think they help much in the case of gravity loads, particularly if there is pattern loading but that would have to be analyzed.
BA
RE: double wood beam - shear control?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: double wood beam - shear control?
I've had good results with Gorilla Glue; is that approved anywhere?
Secure the top and bottom caps with a crapload of screws while the glue cures; that way the screws 'only' need a grip length of 7cm.
Mike Halloran
Pembroke Pines, FL, USA
RE: double wood beam - shear control?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: double wood beam - shear control?
RE: double wood beam - shear control?
RE: double wood beam - shear control?
They say thats the timber grade they have - investor has his own woods so he prepered own beams.
Usually I choose C24.
RE: double wood beam - shear control?
Epoxy and split ring connectors.
http://www.awc.org/pdf/codes-standards/publication...
Plus a bunch of bolts.
If there is no flashing cap on the top surface, lag screws from the bottom so the top surface remains intact.
RE: double wood beam - shear control?
Your bending moment and shear diagrams are wrong. It appears to me that you have entered the wrong support conditions into your program.
BA
RE: double wood beam - shear control?
I used pinned supports for timber columns (vertical and horizontal support + free rotation)
RE: double wood beam - shear control?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: double wood beam - shear control?
What load are you assuming in your design? Are you considering the possibility of unbalanced live (snow) load?
BA
RE: double wood beam - shear control?
Here are diagrams for all forces including columns. Also deflection diagram.
I think my model is alright. And yes, snow is considered.
RE: double wood beam - shear control?
BA
RE: double wood beam - shear control?
RE: double wood beam - shear control?
Also you could do this (tension+compression): Picture