Internal RSJ deeper than Joists

[richo106]

I am just going through our joists and beams etc...

 

The SE spec'd an internal beam than spans a few walls inside as 254x102x25 UB 

 

However the posi joists have been spec'd at 225mm, the hangers that are coming with posi joists fit flush with bottom of the joists so the beam will protrude around 30mm

 

What is the best way round this? Can I contact my SE and see if he could spec an alternative beam?

 

I have attached the beam layout and highlighted the beam in question

 

Any help on this would be very much appreciated

 

[Moonshine]

21 minutes ago, richo106 said:

What is the best way round this? Can I contact my SE and see if he could spec an alternative beam?

 

Absolutely,  I would be going going to try and get a beam 10-20mm smaller than the joists to run services under it

[JohnMo]

Mine came with same oversized steels.  I wanted more insulation in the ceiling than the depth of the posi joist/rafter, so counter battened out with 2x 50x50mm battens.  That way I had about 70mm of insulation under the steel to reduce any cold bridge.

[Gus Potter]

10 hours ago, richo106 said:

What is the best way round this? Can I contact my SE and see if he could spec an alternative beam?

 

Can't see that well from your drawing but here is how you can do a qualitative / initial comparison to see what other size of beam might work. I'll use the values out of the Blue book as it's easy, just watch out as I'm using the Eurocode version as the beam axis notation is different between the Eurocodes and the British Standard.

 

https://www.steelforlifebluebook.co.uk/ub/

 

First thing we look at in this case is how "bendy" the beam is.. how much it will deflect. The amount of delfection is proportional to the beam stiffness about the axis it is being bent.

 

We go to the section properties part and look up the second moment of area about the y-y axis for a 254x102x25 UB  which gives us a value of 3410cm⁴

 

Could we use a 203 x 133 x 30kg UB.. it does not have a larger second moment of area (only 2900cm⁴) .. we could check it but we are wanting to know quickly what could work.

 

Lets look at a Universal column section and pick one that we can probably get off the shelf.

 

https://www.steelforlifebluebook.co.uk/uc/ec3-ukna/section-properties-dimensions-properties/

 

A 203 x 203 x 46 kg UC has a second moment of area of 4570cm⁴ .. yes that will be plenty stiff enough. But it is nearly twice the weight but it could well get rid of the downstand.

 

-------------------------

 

Next we check for strength. We don't want the beam to fail by the steel yielding.

 

Moment capacity of a 254x102x25 UB = 109 kNm

Moment capacity of a 203 x 203 x 46 UC = 176 kNm yes the UC certainly looks strong enough.

 

Now if we can live with the extra weight of steel we can go to the SE who will check loads of other things; the buckling strength not least, which could less depending on how the joists are to be supported, and make sure it can be properly supported and so on.

 

You also have a larger distance from the edge of the flange to the web so you need to check again how the floor joists and so on are connected.

 

We could explore further. Would a 203 x 133 x 30 UB beam work if we welded some flange plates to it.. maybe but the cost of the plates and welding could well outweigh the extra weight of the UC section. Could we use a couple of back to back channel sections.. maybe but channel sections are more expensive and you need to bolt them together. 

 

Hope this helps but ask your SE and check what fire protection you need to the steel and how this is to be achieved. Sometimes a downstand in the ceiling can work well in big open plan areas as it provides demarcation between different "zones" you may want to create while still having the open plan effect.

 

 

 

 

 

 

 

 

 

 

 

[George]

Ah, 203UCs, saving SE's bacon for generations.

 

Two other options that may work:

• A pair of 203x133x25 UBs - similar steel weight to the 203UC but easier to handle on site, minimal extra fabrication but may be too wide - although I think there is scope to just shift the beams downwards on plan
• If there's a wall above to push the 254UB up into it (but I suspect there's a doorway somewhere along the beam span so that's no good)

 

Anything else requires excessive fabrication/cost as Gus says. 

[richo106]

Having spoke to my SE, he has reviewed this beam and has now specified 203x133x25 UB

 

Thanks again for all the replies

[Moonshine]

22 minutes ago, richo106 said:

Having spoke to my SE, he has reviewed this beam and has now specified 203x133x25 UB

 

Thanks again for all the replies

 

good to hear, though are you Posi joists on site? you will need to accommodate the slightly wider beam with the joist layout. 

[richo106]

3 hours ago, Moonshine said:

 

good to hear, though are you Posi joists on site? you will need to accommodate the slightly wider beam with the joist layout. 

Not yet, I have not ordered them yet. So will be making amendments to the lengths etc

 

Thanks for the heads up

 

 

[Nickfromwales]

225mm posi’s leave very little space for services, so beware. 
If you look at the chord depths, and discount 2x from the overall height, you’ll see that you’ve around 120mm or less, and a soil fitting knuckle will just about, or if less won’t, fit in there.

Have you planned your M&E layouts / large bore pipe runs yet ( FW & MVHR ) ?

Leave the steel alone and get deeper joists would be my advice, unless your services are boxed in above / below the joist voids….

[Gus Potter]

21 hours ago, George said:

Ah, 203UCs, saving SE's bacon for generations.

Too right!

 

21 hours ago, George said:

Two other options that may work:

• A pair of 203x133x25 UBs - similar steel weight to the 203UC but easier to handle on site, minimal extra fabrication but may be too wide - although I think there is scope to just shift the beams downwards on plan
• If there's a wall above to push the 254UB up into it (but I suspect there's a doorway somewhere along the beam span so that's no good)

Great options from George.

 

9 hours ago, richo106 said:

Having spoke to my SE, he has reviewed this beam and has now specified 203x133x25 UB

Spot on, must have been some spare capacity in the design.

 

All the best with the project.