Foundation Design

There are some new and interesting developments in foundation design which offer an alternative to the way we have been constructing foundations for the last few hundred years. At least so it seems. It would be interesting to look at these new developments in the context of the traditional ways of supporting a building.

Lets consider what we expect from a foundation. It has to be able to spread the load so that the ground can support the load. It has to be stable, so that it will not move around. Sometimes the foundations are used to anchor the building so that it will not overturn – this is particularly true of taller, lightweight structures such as timber frames houses.

Image courtesy of Wiki Commons stock images.

 

So how do we achieve this? Spreading the load is not difficult except where the soil is very soft, and that is not usually the case in our area. Ensuring that there is no movement is more difficult as our clay in the South East of the UK is prone to shrinkage and heave caused by changes in the moisture content in the clay. For this reason, the minimum founding depth is usually a metre and much deeper if trees are nearby. Holding a building down sometimes has to be considered but by the time you have dealt with the other criteria, this holding down or overturning aspect can be shown to be resolved.

The way we spread the load of the building can be dealt with in a number of ways. The usual way is to dig a trench, fill it full of concrete and then build the load-bearing walls off of this. This is called a trench fill foundation (or footing as builders like to call it) where the concrete almost comes to the surface, or a strip foundation if the trench is only partly filled with concrete and then masonry is built up to the ground level. Sometimes we dig a series of holes which are filled with concrete and then beams span between. These ‘pad foundations’ as we call them require less excavation and soil to be taken from the site, and less concrete, but require additional structural elements above.

The above techniques account for 90% of low rise buildings in the UK whereas for the remaining 10% the solution is usually a piled foundation. Crudely, piles are either driven in or a hole is drilled in the ground and then filled with concrete. The piles will give intermittent support just like the pad foundations mentioned above, and so beams have to be used to span across the top to support the buildings walls. Where the hole in the ground is first created and then filled with concrete it is classed as a replacement pile, and where a steel element is driven into the ground it is called a driven pile. Further sub categorisation is made and they are described as short bored or deep bored piles.

Now this neatly brings us onto the first new innovation in the UK for many years. We now have a worm-screw type of foundation which could be described as a large steel screw and this is screwed into the ground where it becomes the support. It reduces the amount of spoil that has to be removed from site and can be installed in any weather.

Another new type of foundation is that promoted by Advanced Foundation Technology Ltd as advocated by Kevin McLoud of Grand Designs. Basically, this seems to rely on removing some soil and replacing it with a material that will not be affected by freezing conditions. I confess to not understanding how this deals with the shrinkage caused by changes in the moisture content of clay and so I will remain skeptical for now, but clearly in areas where the ground is affected by changes in temperature only, this could be effective.

Clearly the type of foundation your building designer or engineer chooses will be based on individual factors pertaining to your project, and the industry is notoriously conservative for not taking up new ideas but it will be interesting to see how these new ideas are taken up.

Written by Tony Keller – Building Tectonics Ltd

Roof trusses

Most modern house roofs use trussed rafters in the construction of the roof. They consist of quite slender pieces of wood, which are fixed together at the junctions with metal plates. The really clever thing about them is that they derive their strength from their geometry, which is always based on a system of triangles. The alternative way to construct a timber roof is for a carpenter to cut timbers on-site, to the correct length, and then nail them together. This process takes longer than unloading the ready-made trussed rafters from a lorry and positioning them on the house. Invariably, the size of the timbers in a cut, or framed up roof, as it is usually called are much bigger than the timbers used in a trussed rafter roof, this is because you cannot rely on the strength of the junctions to transmit the loads in the same way. In a trussed rafter, the metal plates used to join the timbers together have protruding ‘teeth’, which are forced into the wood by a press, and because this is done in a factory, the quality can be more precise than if you were to rely on a carpenter hammering nails in on the building site. Also, the wood used in trussed rafters is selected to ensure that the design strength is achieved. Apart from the economies in wood, and the time taken, the other significant factor is that the trussed rafter can often span considerably further than a traditional framed roof, often from one outside wall to the other outside wall, meaning the internal walls may not be load-bearing, which in turn means less foundations need to be constructed. All of this can result in large savings in house building costs.

There are of course some disadvantages. The resulting roof space will be a bit more cluttered due to the timber members forming the triangular geometry, and the trusses have to be handled and stored with care, as they can be easily be damaged. They can be more easily affected by wood rot than a cut roof because the timbers are smaller, however, it is now usual practice to have trusses treated with wood preservatives, which help to resolve this issue. In the past, the metal plates have also been known to suffer from corrosion, but these too are now treated to stop them from rusting – it may surprise you to know how the damp the internal space of a roof space can be, which is why we now ventilate roof spaces.

Another disadvantage of a trussed rafter roof is that they are usually (but not always) more difficult to convert to a useful space, like a bedroom. Here at Building Tectonics, we do the design for many loft conversions, and we have derived techniques for both types of roof. but generally a little more steel work is required for a roof consisting of trussed rafters. Of course, with a little forethought from the house builder, this can be overcome by using what we call “room in the roof trussed rafters”. These are more expensive than the alternatives, but still a lot cheaper than constructing the roof on-site out of timbers. It is a shame that more developers do not use the room in the roof trusses as it would allow many houses to later be given an additional bedroom more easily if they ever wanted to convert the space, but of course, house builders want to maximise their profit. Self builders should really consider spending the bit extra to give themselves that flexibility later. We usually do recommend this to clients, we have also had some past conversion/alteration projects where because of the drastic nature of the work, we have suggested that while the client is going to all this effort, they may as well remove the existing roof and replace it with room in the roof trusses. Even though taking a roof completely off is not for the faint hearted and can only be done after much preparatory work such as creating a temporary tent over the house, it is sometimes a brilliant success, it is also more often than not, a lot cheaper than many alternatives too.

The trussed rafter roof was a major innovation in the construction industry when they started being used more generally in the 1960’s. Once the early issues of rot and some manufacturing problems were overcome, the only real problem we are left with in their use is that they are not being handled on site with the care that they should be. They create a very strong roof when it is complete, but the slender nature of the wood elements makes trusses very susceptible to damage until they are in place, therefore good on-site management is required. Apart from the framed roof, and trussed rafter roofs, there have been some other types of roof structures formed in wood, one such system is often called a Trussed roof, this is where timbers are bolted together to form a very strong element, which is then used to support purlins and rafters, which can be smaller than those used in a corresponding framed roof. These were used a lot in in the post housing boom of the 1950’s, but even though they were more economic in timber (meaning that there was less timber involved) than the traditional framed roof, they still lost out to the even more efficient Trussed rafter.  However the terminology can cause some confusion when discussing older buildings, but since many (younger) builders have never seen such a thing, getting the terminology correct is now less important.

This is what one happy client wrote after we advised him of the benefits of using trussed rafters:

Thanks Tony for recommending I use prefab roof trusses on my loft conversion and extension. Not only did it give me the ability to have a much wider open plan kitchen family room but it was significantly cheaper. My builder quoted me 12k but I ended up paying just under 7k.

Written by Tony Keller – Building Tectonics Ltd.

Plywood

Plywood is a simple, but effective material. This is used a lot in the world of construction due to its strong structure, but light weight. It’s quite a flexible material, not physically but in the sense that you can add many layers and make it as thick as you need.

Modern plywood as we know it was invented in the 1800’s. The famous two seater aircraft of World War Two, De Haviland Mosquito was mainly made of different grades of plywood. Furniture makers and Architects started using this material around the 20th century after the second world war when production of plywood became a bit more commercial, and also due to its economic value.

Plywood’s strength comes from the way in which it is made.

It is made up of different layers of veneer, with the inner layer being called the core, and the outer layers being called the faces. The number of layers included is always odd so that the faces look the same. This is good for objects which you’d see both sides of, like a door as it gives the illusion of it being one solid piece of wood rather than lots of layers stuck together.

Each layer is turned at a 90 degree angle from the last, this strengthens the structure of the product because the grains are going in different directions. It is then put together using glue. The type of glue depends on where the plywood will be used:

• Interior – this type of plywood could be glued using either highly resistant glue which is fairly resistant to moisture in the air, or intermediate glue which is resistant to mould, bacteria and moisture. Neither should be used for exterior plywood though.

• Exterior – This plywood is made using waterproof glue.

The plywood is then placed between two large hydraulic shelves and squeezed together whilst being heated to dry the glue, it can then be trimmed and sanded to give a better finish. The outer layers can be made of more expensive layers of wood to make it look better as they are usually thinner.

The more layers of veneer you have in the plywood, the stronger the structure will be and the more resistant to impact it will be, so that it won’t split, chip, crack all the way through or crumble. These layers also make it more resistant to shrinking, warping, twisting or swelling so much as ordinary wood.

You can get exact sizes and thickness’s when you buy plywood, which is handy if you’re on a budget, you’d only be paying for exactly how much you’d be using rather than having any waste.If you do buy plywood, be sure to look out for the initials APA (American Plywood Association), or DFPA (Douglas Fir Plywood Association) as these two companies represent most of the plywood manufacturers and test all plywood to ensure that the quality is to a high standard.

Written by Jade Turney – Building Tectonics Ltd.

Plasterboard

Plasterboard is one of those clever materials that seems to get overlooked a lot of the time. It has revolutionised the way in which buildings are built. We thought we would send out a reminder in the form of a blog as to why it is such a useful material.

Plasterboard is made up of an inner layer of Gypsum (which is made up of crystals containing a small amount of water) between two outer layers of lining paper. Different additives can be added to the inner gypsum layer and you can vary the weight and strength of the lining paper, which in turn will give the finished board different properties. For example, standard plasterboard should not be used for damp areas, like bathrooms or kitchens, but you can have silicon additives added to the core to make it suitable for those areas.

One of the properties of plasterboard which makes it so useful is the fact that it is fire resistant. If a fire were to occur, fire resistant plasterboard would give you extra time to get out of the building (up to around 30 minutes) by slowing down the rate at which the fire spreads at.That would definitely help give you those few extra moments needed to get out of there! As well as being fire resistant, plasterboard is also sound resistant. So it can cut down on airborne noises such as speaking or music. Add to these points that it is lightweight, so if you had a plasterboard ceiling for instance, and there was an earthquake, if the ceiling came down, the plasterboard would not cause so much damage.

Most plasterboard has one ivory side and one brown side, and the liner on the ivory side is specially designed for plastering. Plaster should not be applied to the brown side. This is due to the differing absorption rates. The paper liners on plasterboard is made from recycled paper, which is a big positive for the environment.

Perhaps now, we are enlightening you all as to just how useful and versatile this material really is! We hope that you’ll not overlook this material quite as much in the future.

Written by Jade Turney – Building Tectonics Ltd.

Flitched Beams

A client has just asked me about Flitched Beams and I thought it might be an interesting topic for others. As alway, I am not going to be too technical and so this is going to be a quick introduction to the wonderful world of the Flitched Beam. As you will know, steel is stronger than wood because it can withstand compression, ( being squeezed ) much more than wood and it can withstand tension ( being stretched ) more than wood. However a long slender piece of steel will still buckle and so if you can stop it buckling under load, its ability to withstand the compressive forces will be greatly enhanced. So, if you bolt a fat piece of wood either side of a piece of steel so that they act together, the steel will take the compressive forces and the wood will stop the buckling of the steel. Most typically a flitched beam consists of a plate of steel about 10mm thick and 150mm deep with a timber plate also 150mm deep and about 50mm wide each side to form a sandwich. The assembly is bolted together with bolts along its length at about 300mm spacings.

In any beam spanning across an opening, the top of a beam is in compression and the bottom is in tension. The wood clamped either side of the steel will help stop the top half of the steel buckling, thus enhancing the strength of the assembly much more than the wood could do on its own. Clearly a piece of steel of the overall size of the assembly would be much stronger but much heavier too. Furthermore, the wood element is easier to fix too as you can screw or nail into it and the assembly can be assembled on site with can be a blessing if access is restricted. Another very useful characteristic is that in a fire steel loses a lot of strength and collapses quite quickly, whereas timber initially burns until the surface becomes carbonized and chars which creates a protective layer. Also the wood does not conduct heat as well as steel, for these two reasons the Flitched beam performs better in fire than a steel beam. To ensure this we often make the timber constituent a little bigger, creating what is known as a sacrificial layer, so that the beam can be exposed to fire for say half an hour without collapsing.

I am not sure when they were first used but I am aware of Victorian flitched beams. I am also not sure where the name comes from and that would be interesting to know as the word flitch is used to describe many ancient items.

Any feed back would be appreciated.

Written by Tony Keller – Building Tectonics Ltd.