Hi everyone,
I am modelling concrete tunnel rings with an opening. The rings are connected by shear dowels that supposed to have a shear capacity up to 300kN.
I have a few questions regarding the modelling technique of this structure. I have modelled a few rings (including the one with the opening). The purpose of this model, is to check whether with the opening, the opened ring and the adjacent rings are stable under the connection of shear dowels with the adjacent rings (whether the shear capacity is sufficient and will that induce too high shear stress in the concrete ring).
Due to the fact that, at the circumferential joints, the tunnel rings are connected by dowels and has some interfacial friction, I separate the rings by a small gap (0.05m) and I model a small beam to represent the dowel using steel with prismatic section (similar cross-sectional area to the actual dowel). The shear dowel is not supposed to have any bending moment in it, only shears. If I do not release the end moments, there will be bending moment in the beams but if I fully release the end moments of the beams, the beams are just like a flexible hinge, no restraining effect at all.
In Staad, is there any possibility to model such condition? I know we do not have any gap elements in STAAD. Although by right, there will not be any physical gap between the tunnel rings, but if we model the rings together like a monolithic tube with the opening, the behaviour would not be so right because the only way of shear transfer is through those shear dowels. If the shear exceeds the shear capacity, actually the shear dowel would fail.
Please see attached model below.
tunnel opening.STD
The way you have modeled with the gap and the small connecting beam is fine. Only thing is, you need to release MY MZ at the start and ends of these beams. Also you have defined spring stiffness KFY and KFZ. Since you have modeled the dowels as is, I would not define any additional springs as such and just analyze with the moment releases. That way you would get the shears in those members as you would get in your actual dowels which can then be checked against the allowable value.
Thanks for your reply Sye.
I have tried to release MY and MZ at the start and ends of those beams (connector) but it will turn them into a hinged connected members with no shear forces in them and is not helping to transfer any forces from the broken ring to the complete ring.
If I do not release them, there will be shear forces and bending moments and it seems like, only fix connection will work. Also, if the gap is bigger, (longer beam) you will see bigger bending moment and shear force in the beam.
In reality, the 'beams' are dowels connecting between ring to ring so there won't be physical gap between them, purely shear transfer interaction.
Do you have any suggestion sir.
Thanks a lot.
If you release MY and MZ there will be no moment transfer but shear would still be transferred as long as you are not releasing FY and FZ.
The beam element connected with both ends released would act like a hinge, does not allow any force to be transferred.