How many acro props ?

[george765]

I am soon going to have a go at installing my first steel lintel. Is there a 'normal' spacing for these ? the span of the opening will be 3350mm.
It's a cavity wall with good access both sides. Would two be enough (each side) for needles ?
George

 

[leew2]

I had a book which recommended 1 pair of props per 900mm (3 feet) of opening width so a 3350mm opening would require 4 pairs (8 props total).
This is assuming you don't have any joists bearing into the wall just above the opening or other unusual loading conditions which may require extra props.

 

[tim00]

Yes, you should be safe at 1100mm spacing. But why not post some pics of, both sides, of the wall in question.? Breakthro's are more than just acrows and removing masonry. In your own inexperienced interest, dont do anything until you've posted pics and picked a few brains.

 

[george765]

thanks guys, very helpfull, I'll try and do some pic's.
In the meantime, I assume you take up the floorboards and put the acro's direct on to the ground (perhaps on scaffold boards ?) ?

 

[joe-90]

The general rule of supporting walls is draw an equilateral triangle above the opening. Only the bricks in that triangle can fall - so the weight isn't massive.

It's a useful tip to remember.

 

[noseall]

We would probably be using two each side for that span.

Spread the load with planks over the floor joists. No need to take up floor boards unless the floor joist configuration is such that you are on one joist as opposed to several.

 

[^woody^]

If the brickwork is loose (crumbly mortar) then it would mean using more props closer together

 

[Symptoms]

george - support each side of wall as others have suggested and no need to take-up f/bds if joists are at right-angle to the wall so spead load on scaffold boards. The advantage of using the 'pin' method is that you can use a block & tackle (supported from the pins above) to winch-up that +3m steel if you aren't mob-handed.

If you don't want to use pins then consider hiring some of strongboy supports to go on top of the acrows, these fit into the mortar joint so 'gripping' the masonry above. A trick we always used pre- strongboy days was to tilt the acrows so the top plate fitted into the mortar joints to grip the wall. The trouble with the pin method is you have to disrupt the wall above when punching through the pin holes and this can cause problems in older building, especially those with lime mortar or other soft mortar.

Oh, and position the steel at the base of the wall to be chopped out BEFORE you fit the props ... you don't want a huge hole supported by the props and be unable to get the steel in 'cos the props are in the way.

 

[george765]

Some really great tips. I just checked and joist are at 90 degrees so I will put acro's on scaffold boards on floor. Based on your answers I will use 3 each side.
It's a 1920's house so the mortar is quite crumbly, I will consider using the 'strong boys' rather than disturb loads of brickwork above, but is there going to be room for the beams (2 of 200 x 100 bolted together with spacers) to be placed inside of the acro's when using stongboys ? they look like they cause the acro's to stand quite close to the wall in question ?

 

[^woody^]

Strongboys should only be used for supporting one skin, and do have to stand close to the wall.

So it can make things tight and awkward

You need to plan the work so that the beam can me manoeuvred and put in place once the wall is out and the props are in place.

This may mean putting the beam at the bottom of the wall before placing the props, and cutting out sufficient wall and bearings so that the beam can be lifted up and in - a beam needs more room diagonally than if it is being moved up horizontally

 

[george765]

Thanks for the help guys. I think i'm ready to give it a go. Great forum this !