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Gus’ Structural Engineering and DIY Part 02

Gus Potter

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Acrow props – is there an alternative that won’t fall down?

Yes there is in some cases. Much depends on your programme, whether you wish to give it a go yourself and how much time you have on your hands.

Please feel free to comment / question. I’ll answer if I can (if not, I hope some other BH members will be able to) and provide more detail.

 

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Photo 07 showing holes for the needles and the first few temporary works studs going in. The base of the studs are sitting on a sole plate fixed down to a temporary strip foundation. Part of the sub floor has been built up and concreted to provide a working platform.

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Photo 04 (copied from previous post)

In my case I have been dipping in and out of our project. I reckoned that I would need some 25 – 30 Acrow props for the temporary works which would take me about 4- 6 months as I was also mucking about with other bits of the build. To hire 25 props at £ 5.00 per week = £ 125 per week!

I could have bought the props but I would have needed to get rid of them after in my case, just gave up on that.  I have two or three old props that were used to help tighten up (pre load / stress) the timber propping.

The timbers 145 x 45 (grade C24) to form the needles studs, top and bottom sole cost ~ £350.00 ex vat plus a few screws (about 250 number  5 x 80 gold screws I think) to fix things together.

I knew that I could recycle the temporary studs to form a new floor we needed and that if I cut back the needles I could also use these to prevent the top steels from moving sideways. This is called simplistically “ lateral restraint”. Also, I used same needles to provide additional what is called “torsional”  restraint to the top beam ,  a slightly different effect, but is it to do with the fact that I used one beam with a top plate to support both the inner and out leaf of masonry which can be loaded in different ways at different times. These different loading patterns cause an uneven loading on the top of a single beam.. resulting in a twisting (torsional) effect. You can see more photos in post 01.

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Fig A 01 showing proposed rear elevation.

How does it work / stay up?

The starting point here is to calculate the amount of load. With this house the roof spans front to rear, the attic is full of stuff, the walls are rendered brick cavity walls and you have part of the weight of the new extension roof that is now attached / built into the outer leaf of the house.

Once you calculate this you end up with a working load you need to hold up of round about 25 kN/m run of wall.

Approximately 1.0 kN (kilo Newton) is ~ 100kg so 25kN/m is about 2.5 tonnes per metre run of wall. Thus if you put your needles at 600mm centres then each cluster of needles has to carry 0.6 x 25 = 15 kN (1.5 tonnes) and the studs supporting the needles at each end need to carry a vertical load of roughly 0.5 x 15 = 7.5 kN (0.75 tonnes).  These types of loadings are as about as much as you can “do” for this type of system. There are other ways but you need more height for timber needles.

To make life a bit more difficult the window lintels on the first floor cause the load from the roof to be unevenly distributed so some needles need to carry more load than others. You can see from the photos that some needles are in pairs, some are a cluster of three.

In terms of the studs these needed doubling up in places and the whole lot horizontally braced at mid height with some diagonal bracing just to stiffen the whole lot up. While this sounds like a lot of work once you get going it’s actually quite quick.

This approach is a little different from using Acrow props. With Acrows you wind up the prop to preload the system so that the upwards force is approximately equal to load the wall you are taking away is carrying.

Aside,  If you are Acrow propping things then be careful not to over tighten the props as you can easily start to lift parts of the house accidentally!

Here, what I did was to use a combination of the spare props I had combined with timber wedges to pre stress the system, one set of needles at a time. The other thing is that over time the timber can shrink / creep a bit so I was careful to get the timber dry first and then monitor the system as I went along.

All sounds complicated but once you grasp the principles a lot of it is common sense and this is not a reinvention of the wheel. I know a few old joiners and brickies that still remember doing this when they were serving their time.

Some extra information that may help. As always, comments welcome.

Options for beams-  In terms of material vs labour cost it can sometimes be beneficial to design the steel beam as a single beam with top plate. The one you see here forms part of a double flat portal frame.

You will also see designs that use a separate frame for the inner and outer leaf, a question of balance, each project is different.  You can introduce a problematic thermal bridge at times if you deploy these single portal principles to an external wall, but in my case the steels are now internal..  the thermal bridging is resolved in the main.

Propping considerations.

Below is a screen shot of part of a temporary propping design for a set of 4.0m bifold doors etc. There was another big chunk of the rear wall to come away at the same time which is not shown. The contractor here was stopped from working until remedial action was implemented.

        

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Fig 02: The purple lines are the props and what is called the lacing / bracing system to stop sideways movement.

Work was stopped when this was discovered. Part of the contractor’s defence was that “he was insured”

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Photo 08

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Photo 09

A few points. The Contractor had not investigated the ground under the slab and the base of props can slide about. The site investigation had revealed this ground was very soft. Use of Strong Boy type supports at the head of the props.. these have a reduced load capacity and not suitable for the loads to be carried in this case. Props well out of plumb, no bracing.. further comments welcome.

On a finishing  note the timber propping scheme may look a bit rough but it was designed and executed with no unexpected movement.  If this approach may suit you then as always check with or be a competent person.

Thanks for reading.

 

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This is really interesting, thank you Gus. To take up your point about being a competent person, is it acceptable to do self calculations for your own extension (for Building Control/Insurance etc if you properly understand and execute the concepts? Your demonstration of a  confident, rather than competent builder is pertinent, as a self builder, I've been guided so far by the principle that if it looks wrong, it it might well be, rusty physics A Level notwithstanding (see what I did there?). I'm guessing the homeowner panicked and asked for a second opinion? In my own field, I've had an overconfident rider get on a difficult horse without a helmet, telling me he was 'insured', as if that would somehow protect him.  

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There were not many notes on the drawing, so the contractor should have read the one that said props no more than 25mm out of plumb!

 

How is the cross bracing normally done?  Scaffold poles?

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9 hours ago, Jilly said:

This is really interesting, thank you Gus. To take up your point about being a competent person, is it acceptable to do self calculations for your own extension (for Building Control/Insurance etc if you properly understand and execute the concepts? Your demonstration of a  confident, rather than competent builder is pertinent, as a self builder, I've been guided so far by the principle that if it looks wrong, it it might well be, rusty physics A Level notwithstanding (see what I did there?). I'm guessing the homeowner panicked and asked for a second opinion? In my own field, I've had an overconfident rider get on a difficult horse without a helmet, telling me he was 'insured', as if that would somehow protect him.  

Hi Jilly.

 

@JillyYes, you can do your own structural calculations and submit them for your own extension but you should be competent. 

 

There are other factors worth some discussion. Here are a couple.

 

The first is to recognise that Building control will make reasonable checks to satisfy themselves that the design meets the Building regulation requirements.  For example, they may check 10% of the maths in the calculations and look at perhaps the stability system. However, it is the responsibility of those carrying out the work to ensure that the provisions of the regulations are met in full.  You could look it another way. If BC were responsible for the design then there would be less of a requirement for an SE say to hold the level of PI cover that they do.

 

Secondly, if you do your own calculations and something goes wrong where do you stand from an insurance point of view? I would recommend that if you are going to do your own calculations then it’s worth having a discussion with your insurer. Before you start.

Edited by Gus Potter
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8 hours ago, Mr Punter said:

There were not many notes on the drawing, so the contractor should have read the one that said props no more than 25mm out of plumb!

 

How is the cross bracing normally done?  Scaffold poles?

 

@Mr PunterYes usually with standard scaffold poles which have an outside diameter of  ~48.3mm.

 

The top part of the prop (inner tube) is the same diameter as the scaffold tube so you can make 90 degree connections with a standard double coupler, angled (diagonal bracing) connections with a standard swivel coupler. The outer tube of the prop has a diameter of ~60.3mm so here you use reduction couplers.

Edited by Gus Potter
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