This is one of the most tricky parts of our job, as one of the most hazardous times in the construction or alteration of a building is when the works are in progress.
As consulting engineers, our brief is normally to design the permanent parts of the structure, and in the traditional contract model the contractor is responsible for the design and execution of the Temporary Works. Thus, the conscientious contractor would employ his own consultant to provide him with the necessary design.
The Building Inspectors are also only responsible for ensuring that the permanent works are carried out in accordance with the specification, so unless the temporary works are an immediate risk to life and limb they are not empowered to enforce a particular method.
Regrettably, in domestic projects the builders are too often inexperienced or unaware of the correct practice for supporting walls and other parts of a building. I am often appalled when I visit a site to see the back wall of a house knocked out to receive a lintel, with no props supporting the masonry but plenty of Acrow props beneath the first floor. As the wall comprises the majority of the load, this is clearly not right!
The argument is often put that the wall will somehow "arch" over the opening, and in some cases, with smaller openings, it does. However, as current projects demand ever larger opening widths, the ability to arch is eliminated by the lack of restraining walls either side, and the effect of window openings in the storey above.
So, if the props are only placed beneath the first floor, the wall can only be supported by resting on the ends of the floor joists. In practice, most joists have a gap above them due to shrinkage as the timber dries out, so the floor will either be jacked up or the load is transferred into the skirting board. In older houses, these are attached to the wall with cut brad nails every 18" or so; in newer houses they may be screwed or stuck on with "No Nails". As a storey height of 9" thick brickwork weighs over a tonne per metre run, there is no way this could be considered safe!
Needle Beams and Props
The "textbook" method for supporting a wall is to punch holes in the wall every 90 to 120cm and insert needle beams through the wall. The ends of the needles are propped, leaving a working space beneath for inserting the lintel. This is reliable and robust, however the common objection to this method is the disruption to the rooms above (one of which is inevitably a fitted bathroom!) and the need to repair the holes afterwards. This has to be the preferred method for the majority of projects, however.
A common method in small projects is to use "Strongboy" heads. These are fabricated steel fittings that fit over the top plate of an Acrow prop to give a projecting spade-like blade. This is inserted into a slot cut into the bed joint of a masonry wall, so that the wall can be supported while the prop does not have to be directly below the wall.
The convenience of this method is undeniable, but due to the limited load capacity of these devices their safety is too often overestimated or taken for granted. The manufacturer's stated load capacity is 340kg, which amounts to about one square metre of brick and block cavity wall. This means that you would need supports every 450mm to avoid overloading the Strongboy heads. For solid 9" brickwork, the supports would need to be virtually touching each other!
In practice, the supports are normally spaced at one metre intervals, and the props are overloaded, causing cracking to the supported wall and bending the blades of the Strongboy heads. I have experienced several cases where unwitting builders have caused extensive damage using this method, and I believe that insufficient information on their proper use is provided when they are purchased or hired.
We can help!
So, if you are in any doubt about the proper method for supporting a building while structural alterations are being carried out, please get in touch with Structural Engineers Cambridge Ltd. We are happy to help.