How many props...

[spfc]

I have some builders doing a single storey rear extension and because I do not want to rely only on their "experience" (last job I did this, last job I did that) I thought I'd post this question here.

They are going to knock through the rear wall of the a mid terrace house to make an opening 4.7m wide. Builders are planning on using 5 props/strongboys for the outside wall and 5 props to hold the floor joists on the inside.

Does that sound about right? I thought the spacing between the props should not be over 900mm or am I talking nonsense?

 

[oldbutnotdead]

Sounds OK to me as long as both ends of the opening are far enough from the return walls and the wall being taken out is in good nick. What's the floor inside like- if suspended you'll need load spreaders on the floor (running across the joists) as well as the ceiling.

Are they looking after building regs or are you?

 

[^woody^]

last job I did this, last job I did that

"I thought you said that you used five props on the last job?"

"We did"

"Then why has my house fallen down?"

"The house fell down on the last job too"

 

[noseall]

Five! Each side!!
Wow, must be one heavy building.

 

[cAtLeYx]

Temporary works - The propping should therefore be deigned.

 

[ree]

"deigned" as in condescend? Or couldn't you quite make it to the word designed?

Are you claiming that the OP's job referred to above needs a worked design for the props?

 

[cAtLeYx]

Yes - The person organising the temporary works needs to be aware of the problems that can occur at each stage of the process and how to prevent these. They need to coordinate design, selection of equipment, supervision of work, checking completion, authorisation to load and removal. Unless this is done in a thorough and systematic way problems are likely to occur. If you take this on yourself you must ensure each part of the process is correctly carried out.

 

[tony1851]

Yes - The person organising the temporary works needs to be aware of the problems that can occur at each stage of the process and how to prevent these. They need to coordinate design, selection of equipment, supervision of work, checking completion, authorisation to load and removal. Unless this is done in a thorough and systematic way problems are likely to occur. If you take this on yourself you must ensure each part of the process is correctly carried out.

All strictly true, of course, but on small jobs they won't go through all that rigmarole - they'll just do it.

 

[cAtLeYx]

Thanks Tony, would we not consider a 4.7m wide opening through the rear elevation of a mid terrace property to be a little more than a small job. We are not taking about a door opening.

 

[noseall]

Yes - The person organising the temporary works needs to be aware of the problems that can occur at each stage of the process and how to prevent these. They need to coordinate design, selection of equipment, supervision of work, checking completion, authorisation to load and removal. Unless this is done in a thorough and systematic way problems are likely to occur. If you take this on yourself you must ensure each part of the process is correctly carried out.

Are you SURE you are in the construction industry? You seem more like a clipboard warrior to me.

 

[cAtLeYx]

Are you SURE you are in the construction industry? You seem more like a clipboard warrior to me.

I don't see you offering any advice to the OP. You only ever question my knowledge without ever offering yours? Perhaps you could answer one simple question directly - What advice would you offer the OP.

Five! Each side!!
Wow, must be one heavy building.

No advice given here.

 

[noseall]

What advice would you offer the OP.

Make sure you stabilise the props, i.e. so there is no lateral sway, until they are tightened.

Make sure that what the props sit upon has no give.

Ascertain the bonding of the masonry when selecting prop distances, in particular how friable the masonry joints are.

Ensure that the needles collect the masonry entirely and if they don't, be sure to pack any voids.

Ensure that if packing floor joists (as needles) that the joists are actually let into the wall and not secured to a plate.

It may be prudent to have the beam in position on the correct side of the props if there is limited room for manoeuvre.

I could go on........

 

[noseall]

No advice given here.

Correct. Looking back at your posts on this thread indicates you are more interested in hi-vis rather than help.

 

[cAtLeYx]

Then please do -

Make sure you stabilise the props, i.e. so there is no lateral sway, until they are tightened.

How would you make sure there is no lateral movement - Install diagonal bracing perhaps? You could of mentioned this? - Lateral movement needs to be designed.

Make sure that what the props sit upon has no give.

How much "give" - Are you referring to deflection of the internal floor how much give is too much. The load will determine the point load on each prop and therefore installing a spreader beam, will dissipate the load. The spreader needs to be designed.

Ascertain the bonding of the masonry when selecting prop distances, in particular how friable the masonry joints are.

Brick bond, triangulation of the loads above. You suggesting we rely on the condition of the bed & perp joists for cohesion?

Ensure that the needles collect the masonry entirely and if they don't, be sure to pack any voids.

Agreed

Ensure that if packing floor joists (as needles) that the joists are actually let into the wall and not secured to a plate

Agreed - As the joist will act as a needle if pocketed into the wall. But we still need to consider the load of wall and now a section of the floor.

It may be prudent to have the beam in position on the correct side of the props if there is limited room for manoeuvre.

Agreed - You must ensure that when carryout your temporary works design you consider this.

Do we know what load will be put onto the propping and therefore if the spacings and diagonal bracing is sufficient to take the load. No - Therefore we need a calculated design.

 

[noseall]

How much "give" - Are you referring to deflection of the internal floor how much give is too much.

Not sure on an amount, but we would double plank any timber floors with our stout boards.
If there was to be unusual point loading then I would consider removing the floor boards and look at packing the joists etc (if there was no solid over-site concrete).

Brick bond, triangulation of the loads above. You suggesting we rely on the condition of the bed & perp joists for cohesion?

No. But I would be wary of loose bricks falling between gaps in props etc. The needles will do the work. The cohesion thing is more of a safety factor so no one gets hit by falling masonry.