Stability walls and cores

[Casparov]

Dear all

We’re designing a concrete structure containing a few stability issues, e.g. stability walls and concrete cores (stairs, elevators). All floor slabs are prestressed hollow floor slabs which don’t really add to the stability of the structure. Instead, the compressive layer (50 to 70mm) on top of those slabs will be reinforced to create a diaphragm which should divert all horizontal loads (wind load) to the stability issues.

There are a few questions:

What’s the best method of implementing the reinforced compressive layer, which should act as a diaphragm. I don’t really need the prestressed floor slabs to be modeled, as the basic dimensioning has already been done and the loads (also dead load) can be implemented as planar loads. However, if so, the dead load of those slabs won’t be used for calculating the centre of gravity (let’s say, of a story), but as those slabs don’t add to the stability, I don’t see a reason why it should?

In the example (see attachments), only two walls have been used. Both walls are directed along the Y direction and we’re only interested in the stability due to wind in the same direction. Wall 1 is 10m long, wall 2 only 5m. Both walls are 300mm wide. The building measures 10m by 10m and is 8m high. Only one story (“Story 1”) has been defined and each wall is appointed to either “Core wall 1” or “Core wall 2”.

The diaphragm on top is defined as Slab-Rigid diaphragm (No finite elements, Two-way load distribution and Rigid body Diaphragm).

Cladding has been added, and a horizontal planar load of 1000 kPa in the global Y direction. Note: the cladding is load bearing in the (global) X direction. However, I’d rather have it only load bearing vertically (global Z direction), as this would be more in correspondence to the reality. But appointing this one-way load distribution to the cladding won’t create any results (i.e. 0kN…). I don’t really know why.

If you take a look at the second attachment, the results created don’t really correspond to what we’d expect: the smaller shear wall isn’t really used. So clearly, something must have been defined incorrect?

If you could help us with these issues?

Thanks in advance for your appreciated advice.

Best regards.

[tony.ridley]

Casparov,

What’s the best method of implementing the reinforced compressive layer, which should act as a diaphragm. I don’t really need the prestressed floor slabs to be modeled, as the basic dimensioning has already been done and the loads (also dead load) can be implemented as planar loads. However, if so, the dead load of those slabs won’t be used for calculating the centre of gravity (let’s say, of a story), but as those slabs don’t add to the stability, I don’t see a reason why it should?

You can define your hollow core slab as shown, this will then include the true self weight in the centre of gravity calculation;  access through panel definition dialogue box.

 If you take a look at the second attachment, the results created don’t really correspond to what we’d expect: the smaller shear wall isn’t really used. So clearly, something must have been defined incorrect?

From looking at the jpeg's you attached the load is being distributed left to right (global x-axis direction), but the cladding is supported on one edge.  So Basically 100% of the load will go to the 10m x 10m wall.  I guess the smaller wall is taking some load via diaphragm action of the slab.  

Cladding has been added, and a horizontal planar load of 1000 kPa in the global Y direction. Note: the cladding is load bearing in the (global) X direction. However, I’d rather have it only load bearing vertically (global Z direction), as this would be more in correspondence to the reality. But appointing this one-way load distribution to the cladding won’t create any results (i.e. 0kN…). I don’t really know why.

I think the reason you get 0kN when distributing load in the global Z direction is because in that case the cladding is only supported on one side (the top).  The cladding cannot have support at the base of the structure as you have defined it.  Why not just use a line load on the edge of the slab if thats all you require?

If you go into view properties, turn on load ditribution, you will be able to see how the cladding load is being shared around.

Hope this helped,

Tony