double wood beam - shear control? 4

[mats12]

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?

 

[jdgengineer]

You would need to connect them in order to develop the shear flow across the joint. If you do not, I would assume each beam only takes the load relative to its own stiffness (i.e. not composite). If the existing loading is not removed prior to placement of new beam, new beam would only assist in taking any additional load. Shear force would be dependent on loading taken by each member.

 

[mats12]

I know i have to have connection so the horizontal shear flow between elements can develop.
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?

 

[msquared48]

For what you describe, no connection would be required to develop the shear force.

I would still stitch them together nominally though considering seismic forces.

Mike McCann, PE, SE (WA)

 

[BAretired]

In order to develop the combined section in shear, you must develop the combined section in bending. If bolts can develop the full horizontal shear on the interface, the beam acts compositely and shear at supports can be calculated based on a section 20 by 42.

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

 

[mats12]

Tnx for answers!

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."

 

[jdgengineer]

You would distribute the load based on member stiffness. Ignoring shear deformations and assuming same material this becomes distributing based on their moment of inertias or in this case the cube of the member depth.
14^3 = 2744
28^3 = 21952

Therefore 14 cm beam takes 2744/(2744 + 21952) = 1/9 of total load.

 

[JLNJ]

Is the load vertical or horizontal? How does the load come to the beam/ how is it tied in at the ends?

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.

 

[n3jc]

What is the lenght of the beam? material?

 

[mats12]

It is simply supported beam with vertical point loads from rafters.
Maximum vertical shear is 40 kN at supports.
Timber is C16.
Lenght of the beam is cca 500 cm.

 

[jdgengineer]

Is this an existing condition? How are you adding another beam on top in this instance?

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.

 

[KootK]

I don't love this strategy.

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.

 

[BAretired]

If the reactions are 40 kN then the total load on the beam is 80 kN.
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

 

[mats12]

The load is not applied yet.
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:

 

[molibden]

Can you put anoher element from the side where it is overstressed? You can glue and screw plywood on both sides of the beam.

 

[mats12]

Unfortunately I cant. Agreement was double wood beam and bolts only.

 

[BAretired]

In that case, you need to determine the number and spacing of bolts required to resist the horizontal shear on the interface of the two beam elements.

BA

 

[jdgengineer]

I'd definitely, prefer to see the beam replaced with a full-height beam, but if that is not possible, I would do as BAretired mentioned. To be honest, I'm shocked that the beam is shear controlled based on the span in the photo. I would expect it to be deflection or bending moment governed.

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.

 

[jdgengineer]

Shrinkage of the wood will also introduce some slip in the connection. It's likely that the bolts will not be tight over time as a result.

 

[jdgengineer]

If aesthetics is not important, you may be better off using LTP clips or something else attached from the side. I think these would be more tolerant to wood shrinkage as it would allow the wood to shrink still outside of the connection. Full-height through bolts don't really give the wood anywhere to go.