Disproportionate collapse - masonry/hollow core 2

[xxpegasus11]

We been asked to look into detailing to meet disproportionate collapse requirements for a building under construction. The building is 4 storey block of flats plus basement, approx 24x24m. External walls are cavity 100brick/75cavity/200mm solid block and internal walls 200mm solid block. Hollow core slabs and stairs have been designed by pc manufacturer for all vertical loads. First two floors of slabs have been erected. I am concerned that no other provision has been made for disproportionate collapse. There will be a screed topping which can be reinforced. Any thoughts, advice by anyone more familiar with connection possibilities would be appreciated.

 

[johnp]

As I understand it you are building walls with two materials - structural brickwork, and concrete block with a cavity. Concrete shrinks and brick swells.

When I faced this problem several years ago I started by contacting the Brick Manufacturers Association - asking information on loadbearing structural brickwork. They were very helpful.

On the otherside I contacted the Concrete Block Manufacturers Association. They publish a helpful book and provide some handy downloads.

Between those I've done a lot of reading building construction books, and looking up magazines like the Structural Engineer UK version.

There are no instant answers - particularly in a field like disproportionate collapse. There are several books published on the subject of structural collapse, but it is difficult to find appropriate previous examples which match the problem in hand.

Most designers just stick to the current Codes of Practise because it can cost big bucks to cater for serious cases of disporportionate collapse.

Johnp.Rz

http://www.mets.net.au

 

[csd72]

I assume this one is in the UK as you are referring to it as disproportionate collapse. In Australia and the US it is referred to as progressive collapse.

If so, look up the building regs (not sure if it was london or UK), they referred to allowing for the removal of a certain area of party wall which had to be designed for.

It is primarily failure of the walls that will cause the disproportionate collapse. You can allow for this scenario by either:
1. providing a continous member under the floor edges that can span across any gap, or
2. incorporating a bond beam at each floor to provide a tie for arching action across any hole (or cantilever action at ends).

As for the floor immediately above any hole (i.e. all floors), you should provide tie bars between the ends of members to provide catenary action if the floor immediately below blows out. These tie bars can be placed in one of the cores at each end and grouted in. The concrete over the core can often be removed for the bar length to ensure adequate compaction of the grout (ckeck with manufacturer). A tension member around the perimeter may be a good idea also.

Remember that disproportionate collapse does not need to be checked for full live loads (check with code, but no more than that for earthquake criteria).

IStructE has recently published a guide book on this matter.

 

[xxpegasus11]

Thank you both for replying.

Csd72 - Yes it is UK and the building regulations do apply. Your comments and suggestions are all correct.

I was looking to provide horizontal and peripheral ties to cater for the tension forces required by the Codes. My problem was detailing these in a way that they would be effective. The first 2 floors are already in place, so slabs rest on the 200mm concrete block inner leaf and wall above it has been built. Similarly with internal walls. So providing continous/bond beams in no longer an option. There was a suggestion to utilise the screed topping but I would not have relied on this to provide any composite action - unless i m missing something. Peripheral ties are allowed to be within ,say, 1m from edge, which would help the problem of the walls being already in place. This would mean providing ties on top of the slabs and then breaking into the cores at places to anchor them- does this sound feasible? or any other ideas?

 

[csd72]

How about a continuous angle around the perimeter for the pereter ties. Connect this to the underside of the hollowcore and design it as a tension member in catenary between supports. You could possibly anchor right through the cores and put a spreader plate at the top(in the screed. This would give you spanning ability across any local breaches in the wall.

You should be able to break out the top of the hollowcores locally over internal beams to install internal ties.

The topping slab idea doesnt sound like it would work to me either.

 

[xxpegasus11]

Yes, that is an idea. I was thinking of something along those lines and looking at tying slabs together with ms flat bar and anchoring into the cores with rebar. The angle idea has some advantages and could be buried in the screed. I also have a ceiling underneath so could hide any fixings.