Underslung joists under new steels

[dpblack]

Hi there,
I'm trying to regularise an unofficially converted loft in a 1920s terrace with standard roof construction. Surprisingly most of it is ok but the floor isn't as it is resting on a spine wall that isn't load bearing. The current floor level is designed to match the top of the truss bottom tie, so is about 8". I can't raise the floor by more than about 15mm from it's current height or I'll fail the headroom limits and the stairs would need redone - a massive job

So my structural engineer has proposed running two steels (254x133x30 UB) from party wall to party wall with a 5m seperation (same as the current fake stub wall under the purlins) and then 200x75 C24 floor joists on 400mm centres. Now all that sounds fine and correct, but he's proposed running the steels over the top of the truss bottom tie so the bottom of the I beam would be roughly at the current floor height. The joists would then be underslung from the steel to sit "very close" to the ceiling below. When I challenged him on this, he just said, go find a competent builder who can face the web of the beam with timber and use extra long leg jiffy hangers. But I've never heard of a jiffy hanger allowing a joist to undersling by more than the joist height. In fact, when I looked at a few jiffy hangers they explicitly say a maximum drop of 75mm to 100mm tops.

Untitled

• dpblack
• 12 Feb 2013
• 1

Anyone heard of this? My structural engineer wants me to pay for his current work before he'll talk to me again but I'm not willing to do so until I know he's not barking up the wrong tree. Is it unreasonable to assume that the structural engineer should have indicated fixings in the design?

 

[freddiemercurystwin]

http://www.sabrefix.com/media/3828/jiffy_hanger_1_.pdf

 

[dpblack]

http://www.sabrefix.com/media/3828/jiffy_hanger_1_.pdf

thanks, but I suspect it's not enough as the 200mm high joint has to pass under the 253mm high steel beam and still provide 45mm of wrap over plus the (unspecified) thickness of timber on the beam to wrap over onto (anyone any guesses) so the hanger is too short unless I can just face nail it with no wrap over? and what about the joist swaying underneath as it has no butting on its face (and we all like that)

 

[RonnyRaygun]

http://www.sabrefix.com/media/3828/jiffy_hanger_1_.pdf

thanks, but I suspect it's not enough as the 200mm high joint has to pass under the 253mm high steel beam and still provide 45mm of wrap over plus the (unspecified) thickness of timber on the beam to wrap over onto (anyone any guesses) so the hanger is too short unless I can just face nail it with no wrap over? and what about the joist swaying underneath as it has no butting on its face (and we all like that)

Call the manufacturers to find out if it's acceptable.

If not, you might need to get some hangers fabricated.

To prevent movement of the hanger you might need to screw or bolt some timbers underneath the beam to give the hanger some restraint.

As for whether your SE should provide these details, it really depends on what was agreed and how much you paid.

 

[dpblack]

Well he's a "friend" of my other half and definitely didn't do a mates-rate on this job, nearly £500! Just thought if you did something "unusual" like undersling a massive 8x3 joist (and in one point 3 of them!!!) then you really should provide some guidance. I'll call the manufacturers as you suggested. BC were not interested and just said it is for the engineer to design

 

[chickenlips]

Another way to do it, would be to wrap the hanger over the timber that will pack out the steel. This should give you a little extra for the drop.

You will need full depth blocking between the joists to provide lateral restraint to the joist, as the hanger flange will be less than 3/4 of the depth of the joist.

 

[dpblack]

Thanks, thats' pretty much exactly what the Strong-Tie guy suggested too though I must confess I'm a bit nervous about only fixing the packing timber to the steel after the joists are attached... but it sounds like the required solution.

Good tip on the lateral restraint... my only issue on having full height block between joist will be that the ceiling rafters will remain in place in the gap between the new joist (downstairs ceilings are remarkably ok, even though they've got a 20mm sag!). I suppose I can notch the block. Is there any requirement on where the lateral restraint needs to be positioned relative to the joist ends? I'm guessing only near the hanger end and that standard noggins will do for elsewhere.

I'm still looking for ideas on how to handle the 3-ply of joists for the stair trimmer. Noboby does a long legged hanger at 225mm width. And besides I'm not convinced that 225mm is necessary, especially since the stairs are currently on 2 150x50s and are rock solid - but hey-ho.

 

[RonnyRaygun]

I'm still looking for ideas on how to handle the 3-ply of joists for the stair trimmer. Noboby does a long legged hanger at 225mm width. And besides I'm not convinced that 225mm is necessary, especially since the stairs are currently on 2 150x50s and are rock solid - but hey-ho.

If you can't get any hangers that wide it shouldn't be an issue to notch the sides of the section to fit into a slightly narrower hanger, as shear will not be a problem.
Depends on what size hangers are available though and would need to speak to SE to confirm.

 

[dpblack]

Incredibly the SE agreed that just having hangers for the outer 2 joists of the 3 with the middle one bolted in later was sufficient... looks like I'm in business.