Concrete Block T beam

[Lyledunn]

Three storey office building. This is looking up at the underside of the first floor. The ground floor is to be a day-time diner. The client wants to leave the slab exposed. I can get it repaired but would not be able to get confirmation of the necessary 60min fire resistance. Anyone dealt with this type of issue before? 

[AnthonyB]

How thick are the beams & boards? It would be very unusual for concrete used as a floor to be so insubstantial to be anything less than notionally 60 minutes. BR128 can give a pointer as to fire resistance if you know the thickness.

[Lyledunn]

Thanks Anthony. The floor is circa 25years and was originally underpinned by a ceiling of unknown FR. Not sure about exact dimensions but blocks appear to be standard 220x110x440. Usual issue when BC asks for confirmation of 60min fire resistance. Perhaps obvious to all but no documentation to confirm. Reluctant to proceed with project without ticking that box! 

[Mike North]

You ned to know what is happening at the junction between the concrete and the steel beams, is there a path for fire and smoke to traverse the floor/celling.

Is the steel frame protected?

It looks like there are some major repairs to complete, and what is there concrete (is it RAAC)

[AnthonyB]

In theory that thickness should achieve 60 minutes, but subject to the various caveats Mike mentions. You are unlikely to find anyone to certify it though so as daft as it is you might need to under draw the lot with a 60 minute White Book plasterboard solution

[Lyledunn]

On 27/06/2024 at 07:23, Mike North said:

You ned to know what is happening at the junction between the concrete and the steel beams, is there a path for fire and smoke to traverse the floor/celling.

Is the steel frame protected?

It looks like there are some major repairs to complete, and what is there concrete (is it RAAC)

Mike, the beams are pre-stressed concrete with dense concrete block infill. Very common construction method and, amongst other advantages, particularly useful for giving a solid platform during construction in terms of safety. In the past there may have been a nod to the fire resistance of the arrangement, but no hard evidence in terms of certification. Things have changed dramatically in Building Control, certification is being demanded for just about everything structural. I have absolutely no dispute with that but it has often been a game-changer in our more recent fit-out contracts. 
We are in discussion with BC regarding the issue. Your point about the gaps between the blocks was raised. There is a 75mm sand and cement screed to the floor above so whilst the complete structure might achieve the R, the E and I are less obvious. 
In any event, we can’t get certification so it looks like we will have to underdraw a 60min ceiling as Anthony indicated. 

[Mike North]

Can't you claim the screed as protection

[Lyledunn]

On 01/07/2024 at 15:24, Mike North said:

Can't you claim the screed as protection

Yes, I could, but I have no formal test documentation. It is being considered by BC at the moment but that can’t be the end of the story. Just because BC are satisfied doesn’t necessarily mean others will. I am aware of technical disputes between BC and Fire and Rescue Service on similar issues. Perhaps the insurance provider might also be concerned.

I have issued instructions to ensure that the ceiling is repaired and all openings and penetrations properly sealed irrespective of whether we end up underdrawing the floor with a 60min ceiling. From an academic perspective, if we do drop a ceiling, any services installed above the ceiling will effectively be above the fire line and any apertures cut in the ceiling for lighting or ventilation will need stopping. That might sound silly, but if it is determined that the floor slab is not suitable, then we should really be installing a 60 min ceiling, then our services then another decorative ceiling. Eh!? Funny world, compliance!

[Lyledunn]

On 06/07/2024 at 13:40, Lyledunn said:

From an academic perspective, if we do drop a ceiling, any services installed above the ceiling will effectively be above the fire line and any apertures cut in the ceiling for lighting or ventilation will need stopping. That might sound silly, but if it is determined that the floor slab is not suitable, then we should really be installing a 60 min ceiling, then our services then another decorative ceiling. Eh!? Funny world, compliance!

Well, that has come to pass! BC insisted on a 60 min ceiling. Certification will be required, but how is that achieved with all services in the void and multiple penetrations of the new ceiling?

[Lyledunn]

Just thought I would provide an update. BC have insisted on a new 60min ceiling. Fair enough. However, the officer conceded directly to the client that the existing concrete slab could remain without further repair. All the services, including ventilation for the kitchen and restaurant areas, air con, electrics, IT etc are to be run between the existing floor slab and new 60 min ceiling. 
It seems to me that that without the repair to the existing slab, the services are not within the compartmentation formed by the restaurant. Would you agree?

[Mike North]

Looks like the pendulum is on the upswing, I just hope that there is no changes proposed to Westminster Abby

[Lyledunn]

12 hours ago, Lyledunn said:
It seems to me that that without the repair to the existing slab, the services are not within the compartmentation formed by the restaurant. Would you agree?

Yes, and so does the BC officer. His comments were misinterpreted by the client. Services partly installed now have to come out so that they are below the fire resisting ceiling!