Using facades as supports for floor vibration analysis

[blihpandgeorge]

Hi

I have a floor with a some modest cantilevers to the perimeter due to offset columns. I am getting slightly higher values of vibrations at the ends of these cantilever, i have looked a few things already such as damping, dynamic E, column stiffness etc. When reviewing the UK Concrete Centre published CCIP-016 - A design guide for footfall induced vibration, under section A3 - 'Predicting modal properties by FE Analysis' it notes that: "It has been found by measurement that the additional stiffness and restraint due to a facade can often be well approximated by vertically restraining the slab at its edges. The same applies at concrete core walls"

Placing supports at the ends of cantilevers immediately seems like cheating, however i can see the thinking that under small vibrations only nominal support is required for a short period of time to change the modal shape.

My question is how often is this done or considered? Have people done this before?

Thanks

 

[patswfc]

I think this benefit of cladding is often used when checking floor vibrations. Similar to assuming that the connections act as fixed rather than pinned for vibration checks.

 

[WARose]

Considering the fact many facades have slotted connections that release movement.....I'm not sure I would count on them for much of anything. (Other than weight.) I can't think of a time when I have considered it.

In AISC Design guide #11, partitions are taken into account as far as selecting a damping value.....but that's about it (IIRC).

 

[JoshPlumSE]

Considering the fact many facades have slotted connections that release movement.....I'm not sure I would count on them for much of anything. (Other than weight.) I can't think of a time when I have considered it.

Keep in mind that we're dealing with very low levels of amplitude. Any friction resistance may be sufficient to provide "restraint".

That being said, I think I do agree with WARose in that I would tend tend to see them more akin to partitions where we might increase the assumed damping. But, I'm in the US and would tend to follow the AISC design guide more closely. If I were in the UK, I might think differently.

 

[STpipe]

It all depends on the overall system, the materials, the connection details etc. I would be hesitant to add a fixed support as I feel it would be cheating as you said. At the same time, this type of analysis allows you to fix everything in the model, so in theory it should provide sufficient restraint against small vibrations. To me the element in question would have to be very stiff to consider it a support (like core walls).

Alternatively, if you know the materials, connection details etc., you could discretely model to account for that stiffness at the edge as opposed to only considering the mass and damping effects.

 

[NorthCivil]

Most facades are constructed as "pin and roller" supports, to allow the building to move around them and not damage the facade (is the theory)

in effect, to accomplish weathering as well as structural stability, these "rollers" are specially designed coupling sections ie "arms and sleaves" with gaskets to keep things airtight.

so the facade will act on the building much like a slip critical connection. With the slip resistance coming from the weathertightness gaskets, which will provide very low slip resistance.

 

[CivilStructural01]

I have a few comments:

1) If you are looking at structure, the façade (non-structural component) does not count. For a variety of reasons, primarily, redesign will be required etc. I am not for using "non-structural" as structural just because you think you need it.

2) What material is the cantilever? Concrete, postension-conc, steel, composite. ?

3) Vibration is something you really need to observe in the field, experimentally to understand. Cantilevers are generally not loaded highly or used in the same way the analysis is done. If the deflection works with a cantilever you should be good.

4) Also, is Vibration required per the Project Specifications? Generally it is NOT. Most likely you need to consider only deflection

 

[blihpandgeorge]

thanks all for comments,

patswfc - thanks, this is exactly the context of the question, for assessing vibrations only and not for any other structural aspects. do you have any familarity with this, including whether you are aware of it being done regularly?
a little more background if it helps. The floor is RC concrete and there is a project requirement for virbation. The cantilever deflections are very low. Agree that increased damping accounts for partitions / furniture etc. However partitions are not necessarily full height and are not a reliable feature that can change in time, whereas the cladding are full height and is always there. I agree the cladding will have slip joints under vertical load, however as noted there is very low levels of amplitude being considered.

since originally posting this, i have found the same comment in the UK guidance for floor vibrations in steel buildings (SCI-P354) and also in 2 in-house technical guidance documents from two large multi-national engineering firms. thus there seems to be a body of evidence to do so. What i havent yet found is someone who has actually done it!

any further comments welcome