sense check twin beam calculations

[johnhenstock83]

evening,

 

this is probably the paranoia in me typing, but may I sense check my RSJ order with you? we're opening up an external wall (300mm cavity, brick/brick) to open up an existing kitchen with an extension. we paid a chartered structural engineer to produce calculations and a diagram, which he did. for some reason, it all "feels" a bit flimsy, but keep in mind I'm not an expert.

 

engineer said we should use a twin beam, 203 x 102 x 23 UB, 4.1m long (3.6m opening + 2x over half overlap on 440mm x 215 x 102 padstone), held together by M12 bolts at quarter distance. this is to hold up an external wall on a typical 1950s council house, low ceilings (2.3m internal) and a typical tile roof. house is a terrace.

 

does this sound right? I'm hoping those two beams together will hold the house, but I don't know why I imagined I would need something much more massive. any thoughts?

 

thanks

[markc]

A 203 beam per leaf for a brick/brick structural wall over a 3.6m opening sounds right. 

[Russell griffiths]

11 hours ago, markc said:

A 203 beam per leaf for a brick/brick structural wall over a 3.6m opening sounds right. 

Does the 23kg a metre sound light. 

[markc]

22 minutes ago, Russell griffiths said:

Does the 23kg a metre sound light. 

As a single I would be questioning it, but as a tied double, terrace house so no racking, pretty standard two storey my gut feeling is it’s ok. Obviously I wouldnt put my name to anything without running the calcs or FEA.

[johnhenstock83]

it is indeed 190kg altogether according to the supplier, that made me question it. I thought these beams were much, much heavier. the beams will rest on a 74cm return one side (30 side wall + 44 return) and 230cm+ return other side (30 side wall + 200 return). the diagram was accompanies by several pages of structural calculations, including a section generated by something called tekla tedds.

 

thanks for your thoughts.

[George]

It used to be that if you put in a giant beam you could forgo the engineer. But Building Control have pretty much defaulted to needing calculations for all structural alterations.

 

However, with the calculations taking into account the actual loading and more accurately determining the deflection, the engineers fees *should* be offset by the reduced weight of steel needed.

Edited March 28, 2023 by George

[Gus Potter]

On 28/03/2023 at 12:29, George said:

However, with the calculations taking into account the actual loading and more accurately determining the deflection, the engineers fees *should* be offset by the reduced weight of steel needed.

Yes spot on looks like good engineering.

 

On 27/03/2023 at 20:20, johnhenstock83 said:

held together by M12 bolts at quarter distance.

These are really important as if you have not followed the SE's instructions to the letter re the bolts you have cause to worry as the beam capacity could be reduced by some 40 - 60% as a guess..

Edited April 14, 2023 by Gus Potter

[Iceverge]

9 hours ago, Gus Potter said:

Yes spot on looks like good engineering.

 

These are really important as if you have not followed the SE's instructions to the letter re the bolts you have cause to worry as the beam capacity could be reduced by some 40 - 60% as a guess..

 

For the eternally curious, how exactly do the beams get held together in this case?

 

Is there a bracket bolted to the steel to prevent the steel twisting or is it simply a threaded bolt that maintains the beams at a constant separation to prevent one being pushed sidewards?

 

 

[Mr Punter]

43 minutes ago, Iceverge said:

 

For the eternally curious, how exactly do the beams get held together in this case?

 

Is there a bracket bolted to the steel to prevent the steel twisting or is it simply a threaded bolt that maintains the beams at a constant separation to prevent one being pushed sidewards?

 

I have seen it where they weld a tube on one beam to act as a spacer.

[Gus Potter]

On 15/04/2023 at 10:07, Iceverge said:

For the eternally curious, how exactly do the beams get held together in this case?

That is a very good practical question!

 

If you have a cavity wall and using twin beams under each leaf then you can use M16 or M20 threaded rod in the web. The rods need to go as close to the top flange as you can so the nut clears the root of the web. You get the web drilled and then align the beams so the holes line up. Weave the rod through and wind on nuts so there is a nut both sides of both the webs... then you fiddle about progressivly tightening the nuts.

Now you can only do that if you have a reasonably wide cavity and beam flanges not too wide as you need to get a spanner in between the beams.

 

OR you can be really clever and bolt the rods to the first beam with a nut each side of the beam turned up tight, measure exactly where the inside of the other beam web needs to be, wind on two nuts and turn them in opposite directions so they lock... a locking nut. Then the second beam can be slid sideways onto the rods and you put the last nut on. But all this needs good planning as there are probably props etc in the way.

 

The other easy to do it is to put a flat plate over the tops of the beams and bolt that down to the top pre drilled flanges. The plate acts like a strut.

 

The objective of both methods is to prevent the top flange buckling sideways.

 

Rods can be a bit of a palaver but if you have set your mind on mitigating cold bridging rods work well in this context as their cross section area is a lot less than flat plates.

 

I'm sure there are other ways to do this? but that is some..

 

 

[Canski]

I like the rod idea. I’ve never thought of that. I normally go with a heavy single UC with plate welded to the top. I get the cold bridging bit. The cavity between 2 beams could be well insulated as well. 

[Gus Potter]

To refine a bit you can buy tubular beam spacers off the shelf. See link say below.

 

https://esteels.co.uk/structural-steel/universal-beam-twin-chs-spacer-touching-separate/

 

But often the walls don't have a consistent width of cavity.. good design when say knocking holes in walls is about anticipating some of the things that may not be built the way you think they are and generating as many "get out of jail free cards" as you can.

 

The next stage is to note this on the drawings so the Contractor who is pricing the job can see that someone has put a bit of thought into it and tried to derisk it.

 

 

 

 

[OwenF]

Ours is 4.2m span, internal leaf of blockwork and external leaf of Yorkshire stone. Supporting a first story gable end.

Both UCs are 203x133 but internal is 25kg/m and external leaf is 30kg/m

 

No bolts between the two were specified, surprisingly. Recall the build clarified with the SE during installation but said they weren’t needed

[markc]

51 minutes ago, OwenF said:

Ours is 4.2m span, internal leaf of blockwork and external leaf of Yorkshire stone. Supporting a first story gable end.

Both UCs are 203x133 but internal is 25kg/m and external leaf is 30kg/m

 

No bolts between the two were specified, surprisingly. Recall the build clarified with the SE during installation but said they weren’t needed

203x133 is a pretty stable section so web restraints not required, when you get tall narrow sections the top flange wants to roll over so fixing the twin beams together is required