Do I need an RSJ?

[mishel02]

I am having an extension built to make my kitchen bigger and I am worried about what the builder has said.

The opening is to be 2.1m taking the whole kitchen wall out and extending into the new kitchen. My builder said I do not need an RSJ as the opening is not very wide and he is going to put a concrete lintel in? Is this correct?



Thanks

 

[jeds]

It really depends on the situation. Whilst it's not impossible to use a 2.1m span concrete lintel if it's an external wall with roof and floor loads then it would be taking the loadings probably beyond the limit of a standard size and type. So then you are into hi-strength rather than standard and possibly awkward sizes - like 140 thick instead of 100 thick and maybe up to a 220 deep rather than a 140 deep. Question is; will the builder calculate the loads and make sure he buys the right type and size or will he pick up the first one off the shelf?

If it is an external wall I would advise you to have it properly designed and calculated by an engineer or surveyor. That way you will avoid problems later and have peace of mind but if you do get a problem then you would have the protection of the PI insurance - which the builder almost certainly won't have. It shouldn't cost you more than two hundred quid.

PS. If it's an internal wall then a concrete lintel should be fine but it still needs to be checked to make sure there is no substantial roof load on it.

 

[live4weekends]

Isn't building control involved in this?

I understood that for new openings, especially of this width, BCO would want to see the specs of the lintel and, if possible, check it has been installed correctly with the correct bearings etc..

Have you submitted anything to building control?

 

[Deluks]

I'd go with a steel, only about £100 more on the job, and easier to lift.

 

[mishel02]

Thanks everyone, yeah building control are involved and it is an external supporting wall

I think I will ask the building inspector as I think the builder is looking for a cheaper option

 

[Blagard]

Just 2.1m span? (less than a patio door) I will put my money on the BC inspector being happy with a concrete lintel providing it is a commercially made and approved one.

Otherwise a suitable catnic or similar should do the job. I always thought an RSJ is only really necessary when the spans exceed normal spans in housebuilding.

 

[noseall]

Just 2.1m span? (less than a patio door) I will put my money on the BC inspector being happy with a concrete lintel providing it is a commercially made and approved one.

Otherwise a suitable catnic or similar should do the job. I always thought an RSJ is only really necessary when the spans exceed normal spans in housebuilding.

Nos so, but pcc lintels are usually a safe bet for standard door sized openings where there are no concentrated point loads.

 

[Blagard]

Nos so.

Noseall
Why not so ?

 

[noseall]

I always thought an RSJ is only really necessary when the spans exceed normal spans in housebuilding.

What would you consider a "normal" span?

Anyhoo, it is not just the span but also the load that is relevant. Point loads or concentrations of weight over a small area can exceed what a pcc is capable of even over a standard door opening.

 

[Blagard]

[quote="noseall";p="1507888")
What would you consider a "normal" span?
[/quote]

Noseall
Perhaps normal is not the best choice of words and typical would be more appropriate. Spans up to 3m are frequent in housebuilding, certainly most homes would have a window opening of 2m or above.

As for the loadings, unless you are using the house for something other than domestic use, the typical loadings are easily accommodated by the variety of commerical lintels available. I'm sure you are aware there are many available for the same span to suit the different loadings typically encountered.

PCC lintels should be approved for certain UDL's as are catnics and the like. Adjusting for any point loads to determine if it's going to be an issue may mean going up a grade of lintel or two so that the off the shelf one does the job.

For this thread we are specifically referring to a 2.1m span and like I said providing it is an approved commerical one - and obviously those approval ratings have to be suitable, then thats fine.

A question for you - Are you going to treat the floor joists of the floor above (currently bearing on the wall to be supported at 450 centres) as a UDL or point loads in:
a. A situation where there is say 500mm of brickwork above the 2.1m lintel
b. the same opening with the joists bearing directly onto the lintel.

Assume for your answer there are no load bearing walls intersecting with the wall above the opening.

 

[hotrod]

Mork calling Orson...Mork calling Orson...come in Orson...

 

[jeds]

It's a common thing that to think that as long as a lintel is long enough it's strong enough. A lot of people think that.

Typical loads for an external wall with floor and roof loads are in the order of 9 - 12 kN/m. The safe working load of a standard lintel is about 6kN/m. So you would need a hi-strength one at least, and maybe a 220mm deep hi-strength to be certain. Question is; will the builder calculate it to be sure, or will he take a standard model off the shelf?

Concrete lintels do fail. A job my brother was working on a few years back a lintel in a stairwell collapsed and a fair bit of masonry came down. Luckily it was overnight and nobody was hurt. And that was a major housebuilder.

 

[Blagard]

The safe working load of a standard lintel is about 6kN/m.

Jeds, not sure what you mean about a standard lintel, maybe just a bad choice or words (I'm guilty of that now and then too). Have a look at the Catnic site as an example. You will find lintels in different grades to suit different loadings. They are all standard lintels (as opposed to specials) serving different duties.

Concrete lintels do fail

Sorry to hear your brother had that misfortune to come across a failed lintel. I would agree that a concrete lintel can fail, but I would put my money on it being damaged (dropped or similar) before it was installed and if the cracks were fine enough, no one would see. I bet your brother has picked up a brick sometime in his life to have it fall in two in his hands for the same reason. Generally a commercially approved concrete lintel is going to be fine so long as it his handled properly.

 

[jeds]

Concrete lintels come in two strength classes - standard and high strength. They have different safe woking loads. Catnic lintels are steel. The OP was asking about concrete lintels.

 

[Blagard]

Concrete lintels come in two strength classes - standard and high strength. They have different safe woking loads. Catnic lintels are steel. The OP was asking about concrete lintels.

Point taken. - Incidently I did not know about the high strength variety as always found when I have used them that the rating for my use was OK and did not need to consider further. Cheers.