Modelling of Ramp on Etabs

[BONILL]

I'm designing a five story parking structure using double T's supported on precast inverted T and L beams. I have modelled the ramps on ETABS as a membrane element with one way load distribution and have meshed it into 1x1 elements. The load it appears is transferred correctly to the supporting beams,in terms of the shear and moment values of the beam. However,there is a large amount of torsion transferred to the beams, which is inexplicable since the model should assume the support of the slab at the top center of the beam and there should thus be no torsion on the beams. I have attached the model and would greatly apprectiate any help I could get on this.

 

[slickdeals]

The reason you are getting torsion is because of the deflection of the ramp slab. You have essentially a 6" slab (150mm) trying to span the entire distance.

As a result, you will have a lot of deflection and corresponding rotation at the supports. This is a classic case of compatibility torsion. (See ACI Chapter 11).

You will have torsion in the L beams due to the way the load is placed on the ledge from the double Tee. That torsion should be designed for.

The torsion that you are getting in ETABS is not realistic. For starters, I would bump up the slab stiffness to match the stiffness of the double tees.

I am getting a lot of warnings running your model. Don't know why, did not check it. I would suggest to be careful if this is your first big model in ETABS.

 

[slickdeals]

Also not sure if I like the way the shear walls are laid out. You have a lot of stiffness on one side (left) and only one wall on the other side.

This building is essentially a cantilever in the direction of the ramp. Consider adding a shear wall parallel to the short direction on the right side.

 

[BONILL]

I've increased the stiffness of the slab to resemble that of the Double T's and I am getting better results but only for the flat area of the structure. The ramp portion is still showing excessive torsion on the beams, even though the beams in the ramp and those in the flat portion have the same length and loading conditions.

 

[BONILL]

Here is a more finished version of the model. I am still getting the same problem of torsion on my beams. I could really use some help on this one.