Timber beam overlap splice
Timber beam overlap splice
(OP)
Hey everyone!
Perhaps you can help. I have this timber beam that has to be spliced since 7 m long elements are not available. I want to make a 1,3 m overlap, that has 2 bolts at each end of the splice so the bolt groups are spaced 1 m apart. Is this thing viable? I need to design the bolted connection and so I need the shear force acting on the bolts. Is shear due to moment here moment at the bolt location divided by the lever arm (1 m in this case)? Have you used this kind of splice before?

I also modeled something similar in SCIA Engineer with two timber beams and two very stiff 70 mm long steel rods (to represent the bolts) that would not deform too much under the load and the results seem to be pretty much confirm the case I was thinking about. The shear in one of the bolts is 3,79 kN and 4,2 kN in the other. Screenshots with moments in two beams (one spliced, on not), shear in beams and shear in the bolts.
Moment diagrams:

Shear diagram for beams:

Shear diagram for bolts:

Perhaps you can help. I have this timber beam that has to be spliced since 7 m long elements are not available. I want to make a 1,3 m overlap, that has 2 bolts at each end of the splice so the bolt groups are spaced 1 m apart. Is this thing viable? I need to design the bolted connection and so I need the shear force acting on the bolts. Is shear due to moment here moment at the bolt location divided by the lever arm (1 m in this case)? Have you used this kind of splice before?

I also modeled something similar in SCIA Engineer with two timber beams and two very stiff 70 mm long steel rods (to represent the bolts) that would not deform too much under the load and the results seem to be pretty much confirm the case I was thinking about. The shear in one of the bolts is 3,79 kN and 4,2 kN in the other. Screenshots with moments in two beams (one spliced, on not), shear in beams and shear in the bolts.
Moment diagrams:

Shear diagram for beams:

Shear diagram for bolts:

RE: Timber beam overlap splice
RE: Timber beam overlap splice
RE: Timber beam overlap splice
Can you sandwich the overlapping section between some steel plates and clamp it together with threaded rod?
RE: Timber beam overlap splice
RE: Timber beam overlap splice
RE: Timber beam overlap splice
RE: Timber beam overlap splice
This is copy and paste of EC-5 Clause 5.4.2 Frame structures , Item 9
− The splice connection has a load-carrying capacity which corresponds to at least the
combination of applied force and moment, provided that the timber members are not subject
to bending stresses which are greater than 0,3 times the member bending strength, and the
assembly would be stable if all such connections acted as pins.
My suggestion would be ,
- The depth 145 mm seems not suitable for span 7m ( choose around for rafters around L/24 )..
- Prefer in line splice
- You may select the splice location where moment is less critical ( say 0.25 L )
- Consider dowel fasteners , Flitch plates
My opinion...
Not to know is bad;
not to wish to know is worse.
NIGERIAN PROVERB
RE: Timber beam overlap splice
RE: Timber beam overlap splice
You are scaring me. I hope you have a competant supervisor who reviews your work.
RE: Timber beam overlap splice
RE: Timber beam overlap splice
RE: Timber beam overlap splice
You realize that if you splice two beams together then it becomes one beam, right? Or maybe you're calling them "span beams" and there are two of them. "two span-beams"... Even then, the splice will make it a single span-beam.
This is going great.
RE: Timber beam overlap splice
First, a screenshot from my Tekla model for the rafter. The splice is highlighted red, the strut blue:
Next, a bending moment diagram of the rafter. Look at the diagram - the strut sort of makes the rafter a two-span beam, doesn't it? (I know the connection slips a bit, and the strut has some axial stiffness, but still, it supports the rafter near the mid-span)
Next, the strut with compression:
And lastly, a picture from the local engineer's handbook about this sort of rafter:
It is really common around here (Estonia) but not so much in the US it seems. The thing with these rafters is, that the horizontal supports must be rigid enough to take the horizontal reactions, otherwise the strut becomes a tie and the rafter is basically a simply supported beam.
Sorry, English is not my native language, perhaps you would call this "strut" some other name.
RE: Timber beam overlap splice
Around here we can get engineered lumber in 7m lengths, I don't see why you wouldn't just go that route.
RE: Timber beam overlap splice
RE: Timber beam overlap splice
RE: Timber beam overlap splice
RE: Timber beam overlap splice
I really don't like the concept of this overlapping connection, but first, the statics. The shear diagram for bolts indicates a different shear value for the two bolt groups. That cannot be correct. The shear for each pair of bolts must be the same for a uniform load.
If the load per meter is w, the reactions are 3.5w at each end. Each member carries w for a length of (7-1.3)/2 = 2.85m and w/2 for the central 1.3m. W = 7w for the beam without splice. Reactions are 3.5w each end.
Each spliced member has a reaction of 3.5w at one end and a moment of (w/2)*(2.85^2+4.15^2)/2 = 6.34w at the splice, so with each bolt group separated by 1m, the shear is 6.34w in each group. In your case, w = 4.42/7 = 0.631kN/m so bolt shear = 4.00kN for each bolt group.
Split rings or shear plates could be used to reduce fastener slip, but eccentricity puts significant torsion in each member between bolt groups which needs to be considered.
I don't like it, but would not rule it out without further calculations. The depth of beam should likely be in the order of 300 or 400mm for deflection control.
RE: Timber beam overlap splice
Something to consider is whether two bolts actually achieves the performance you expect - I'll elaborate below
Your scenario has two pieces of overlapping timber with two bolts at the front, and two bolts at the back
This is idealised as a pin-pin support so resolving the moment between front and back as vertical reactions/bolt shears
Imagine that you now remove the back pair of bolts - what happens? Is it theoretically unstable?
Well, the two bolts at the front have some ability to form a moment couple, so it's 'stable' (though with a low capacity)
This highlights the concern that I have with such details
The idealised pin-pin support is not quite correct as you actually have a moment connection at the front and back of the splice
When you try and rotate the cantilever portion of the rafter about the front bolts, they act to try and restrain that rotation as a cantilever/fixed connection - not the pin-pin vertical shear couple you've idealised
This will cause locally high shears either parallel or perpendicular to the grain (depending if you stack your bolts vertically or horizontally)
As these shears increase, the timber will locally crush around the bolts, allowing some rotation and forming some magnitude of pin-pin vertical shear couple as you've expected
However, if too much crushing is required to fully mobilise the pin-pin model then you will instead cause local failures in your timber around the front bolts
We tried to research this and couldn't find any literature on the topic to understand how severe this effect is in practice
We then resorted to building some dummy models relevant to our situation and weren't convinced that the required rotation could be safely sustained
We then tried to build a representative model on the squat rack in my garage...we managed to break a piece of 2x4 but I can't claim that we actually understood anything useful from that (besides, timber go bang loudly...)
RE: Timber beam overlap splice
Regarding material length - specify it as a glulam beam or finger jointed timber beam which will get you over 6 m.
RE: Timber beam overlap splice
RE: Timber beam overlap splice