Drop Panel Sizes

[MHconc123]

This question is primarily in relation to BS 8110-1 but would obviously be applicable everywhere although presumably dealt with a bit differently.
When sizing a drop panel, (which will need shear reinforcement within it) is it necessary to size it based on the point when no shear reinforcement is then required? i.e. once a checked shear perimeter passes with the concrete and longitudinal reinf alone?
Obviously then the slab depth would need to be checked at the drop panel perimeter and onwards as required.
I was wondering if there is any way of assessing the shear reinforcement required if the drop panel depth is kept to a minimum. i.e. if say it was within requirements to work with shear links at the drop panel perimeter and the adjacent slab also work if links were provided.

 

[rapt]

MHconc123,

Drop Panels should extend L/3 (L/6 each side of the columns).

Yes, punching shear checks should then be carried out at the face of the drop panel as well as face of column.

 

[MHconc123]

Thanks Rapt.
The drop panels are all sized as a minimum on the figure you mentioned.
Checking the shear perimeter at the edge of the drop panels however links would still need to be provided.
Then checking the reduced depth of the slab from the drop panel onwards links would be required here also.
The question is really how to assess the required links to be provided between the shear perimeters checked in these areas where the effective depth varies.

 

[rapt]

MHconc123,

Where is htere varying depth, I thought it was either the drop depth of the slab depth?

BS8110 requires checks as 1.5D and then every D from that point until you need no punching shear reinforcement.

It is starting to sou=nd like you need a thicker slab r\ather then drop panels, or even beams!

 

[MHconc123]

Sorry I am probably not explaining this very well.
The depth varies at the drop panel/slab depth junction.
The problem would be how/where the shear perimeter would need to be checked between the 2.
In an ideal situation the drop panel would be sized until no links are required and then the slab checked from the drop panel perimeter to see if any links are required in the slab.
The checks are at 0.75d from the first perimeter at 1.5d.
To assess the possible punching shear links required in the zone of the drop panel/slab junction I am not sure a) where the check should be taken i.e. 0.75d from drop panel perimeter using d of the drop panel?
b) what depth to allow for. At the drop panel depths the shear is always less than 2vc for the slab so based on 8110-1 reinforcement should be provided.
Ideally I would love to simply thicken up the slab but it isn't really an option for this project.

 

[asixth]

What is the design load on the floor if you require shear links in your slab at a distance L/6 from the face of your support?

 

[MHconc123]

I realise this is a slightly odd situation but the main loads are point loads from the supported columns over, not a UDL on the slab itself.
These are being deducted once they are fully within the perimeter being reviewed but as some are a distance from the supporing columns in a few situations the shear perimeters outside the drop panel still exceed vc.
This isn't helped by the fact the slab is over 800mm and hence the distance for the shear perimeters to be checked are further from the column faces than would be for a typical slab.

 

[hokie66]

In a transfer slab like this, don't forget that you have punching shear issues under the supported columns as well as in the slab at the supporting columns.

This is not an "odd situation", but rather is common in thick transfer slabs. If you don't know how to proceed, you need to consult with someone who does. This is obviously a substantial multistorey building to require an 800mm transfer slab, and these type designs should not be done by the uninitiated, or indeed with only the help of your friends on the internet.

Is the slab post-tensioned? If not, it probably should be.

 

[MHconc123]

Hokie66,
I realise the columns over need to be checked also thanks.
I can assure you I am not basing any design on 'only the help of firends on the internet' but was just trying to get some other people's input on accurately analysing a situation that I am unable to find any concise literature on and am working through myself currently.
I have over 10 years experience in RC design and several colleagues with over 30. I personally however have not had a transfer slab of this depth previously and hence was after any guidence available regarding determining and modelling the shear at the junction of the drop panel and the slab (rather than just increasing the size of the drop panel)
The client currently does not want PT although we will be advising they look at the copst benefits of doing so.

 

[hokie66]

MHc,
Sorry if I sounded dismissive.

I am not familiar with the British code, but for shear checks, you use the effective depth of the slab at the location of the check. So within drops, use the drop panel depth, and beyond the edge of the drop, use the slab depth.

It would be highly unusual to provide stirrups outside a drop panel or remote to a column. More depth is the typical solution, but you also may want to consider using higher strength concrete. Prestressing assists as well, although I am not sure how your code addresses this benefit.

How many storeys are you supporting on this transfer slab? Tensioning the slab would allow you to progressively balance the deflections as the loads come on.

 

[rapt]

MHconc123,

I would do checks from 1.5Ddrop from the face of the columns and then 2.5Drop etc until you reach the face of the drop.

Then I use 1.5 * Dslab from the face of the drop panel and then 2.5Dslab etc until it all works.

But I do not like punching shear reinforcement. I normally provide more or better concrete to make punching work.

 

[MHconc123]

Thank you all for you input.
I cannot find or justify 100% an acceptable method to definitely determine the shear at the junction of the drop panel and the slab.
I have therefore increased the drop panel sizes slightly to ensure that the final checked perimeter in the drop panel (i.e. where links are not longer required) is within the drop panel. Then I have checked the drop panel face and the next perimeter into the slab as I would usually do. Although this hardly ever required any rshear reinforcement.

Rapt, out of interest what is your specific objection to punching shear reinforcement?
In the UK it is fairly widely used in flat sl;abs to eliminat the need for drop panels.
Is your concern regarding it being installed on site correctly?

 

[rapt]

MHconc123,

Detailing and Installation, especially in thinner slabs. I think they allow punching shear reinforcement in slabs 200mm thick and above in UK. Taking off covers etc, your effective depth of shear tie is probably only about 100mm or possibly less. I still do not believe it can be placed to ensure that the shear crack is going to cross a vertical leg effectively!

I think factors of safety play a large part in it working in practice.