How to Prop Correctly?

[liftfan]

Hello,

I'm moving a steel into the ceiling void. Is it ok to prop under the floor joists, then have needles in the room above supported on the propped joists?

In more detail:

I have an existing steel which supports a triple joist, which in turn supports a masonry wall, which in turn supports some roof structure (struts).
I'm removing the wall the steel currently sits on. The steel is going to be replaced by a 4.7m long steel in the ceiling void and will rest on an existing steel at one end instead of the wall.

I want to minimise disruption in the rooms above but I think I'm going to have to needle through the masonry wall above to support it. Rather than punch through the ceiling with the props can I just support the joists then use Prop pals above on the joists as needles through the wall (see the blue in the picture below)?

 

[noseall]

Hello,

I'm moving a steel into the ceiling void. Is it ok to prop under the floor joists, then have needles in the room above supported on the propped joists?

Needle4

• noseall
• 16 Apr 2021

Needle 3

• noseall
• 16 Apr 2021

Needle1

• noseall
• 16 Apr 2021

Needle 2_LI

• noseall
• 18 Apr 2021

 

[liftfan]

Thanks Noseall. I didn't realise doubling up a 4by2 would do it for the needles! What's happening inside? Are the timbers on the joist with the joists propped from below?


Here are the structural engineer numbers for the replacement steel:

Floor = 0.4m x 0.5kN/m2 - Dead full UDL 0.2 kN/m
Floor = 0.4m x 1.5kN/m2 - Imposed full UDL 0.6 kN/m
Wall = 2.5m x 2kN/m2 - Dead full UDL 5 kN/m
Ceiling = (6m/2) x 0.25kN/m2 - Dead full UDL 0.75 kN/m
Ceiling = (6m/2) x 0.25kN/m2 - Imposed full UDL 0.75 kN/m
Roof = Struts closely spaced and considering wall spread, load are distributed evenly as a UDL on the beam = 4.2m x 0.8kN/m2 - Dead full UDL 3.36 kN/m
Roof = Struts closely spaced and considering wall spread, load are distributed evenly as a UDL on the beam = 4.2m x 0.75kN/m2 - Imposed full UDL 3.15 kN/m

Added up that's 13.81 kN/m over a 4.55m span so 63kN which I make 6400kg to support.
However, during the job I reckon I can discount the floor loads (it wouldn't be attached!) and the imposed loads (empty rooms and loft) so that gives 4200Kg.

I was looking at Proppals which are rated at 1050kg so 4 would give the 4200kg.
So 4 Proppals supported by 4 acrows directly punching through the ceiling and on the other side the Proppals on the joists supported by acrows below.

Any thoughts?

Thanks.

 

[noseall]

What's happening inside? Are the timbers on the joist with the joists propped from below?

Correct.

 

[^woody^]

Vertical 4x2 has more capacity than horizontal - especially when props are far out and needles long.

How many needles/props are required will be determined by the type and condition the the wall being propped.

Core drilling the needle holes is much less disruption and risk of masonry dropping. And the cores go back in afterwards.

Strictly, the engineer who designed those beams is responsible devising the method that the building is propped and the beams inserted, and they can't abstain from or delegate that responsibility.

 

[noseall]

Core drilling the needle holes is much less disruption and risk of masonry dropping. And the cores go back in afterwards.

Masonry dropping on a 120mm wide hole? Do you use circular needles these days woody?

P.S. the big hole on the left is where the doorway is going and was belted out much earlier to determine floor level, hence not bricked back up.

Core drill, lol.

 

[liftfan]

Think I've found a square core drill:

Not really!

 

[noseall]

Think I've found a square core drill:

View attachment 304414

Not really!

Lol.

Just belt a modest hole out with a SQUARE top to it so that you can hammer wedges in. Woody was pizzed when he wrote that about coring. You can see the hammered-in wedges in my photo.

 

[^woody^]

Masonry dropping on a 120mm wide hole? Do you use circular needles these days woody?

You'll notice a lot a bridges using the same principle.

 

[liftfan]

I ended up using Prop Pals in the end from a local hire company. A brilliant design by a British builder. All worked a treat. No cracks!