Andrew Collinson explains the role of a structural engineer.
Interview with Andrew Collinson
Andrew Collinson is a structural engineer based in Herefordshire. His degree course in the subject was mostly based on theory, so his real training came from learning on the job for a big contractor. From working on site he moved to the design office and this background gave him a good understanding of the practicality of a design.
“The basic job of a structural engineer is to make things stand up”
The engineer's involvement will usually begin with receiving architects drawings, and their job is to make sure the foundations are suitable, and any beams, floor timbers, roof timbers etc in there are the correct size. Quite often they will be appointed through the architect who already has the building design but just needs engineering help in certain areas, like the foundation details or beam sizes. In these cases Andrew will be dealing with the architect directly and may not need to meet the client.
Most people don't go to the expense of having a site investigation
So for most jobs the ground conditions aren't fully known before work commences. The structural engineer will have to make a best guess that will cover most circumstances, by writing down the assumptions that the ground is likely to be something suitable and that particular foundations will work with that. If, once the contractor starts digging, they find anything outside those parameters then they will come back to the engineer for a revision. Andrew says that in most cases standard foundation solutions will work, but there will always be a few that don't, based on whatever surprises are found once digging starts.
“So it’s that thing, until you dig in the ground you don’t know what’s under there so you either pay up front to get someone to come and have a look at it, or you take the risk when you dig that there might be a slight delay.”
And it's important to remember, that everything can be resolved – it just may come with a heavier price attached!
Most cases are covered by Part A of the Building Regulations
Part A of the Building Regulations has a series of descriptions, and depending on the loading of the wall coming onto the foundation, it stipulates the minimum foundation width and how deep they need to be. Andrew says that most people could read off these tables if they knew what the loading coming down the wall was. There are variations for example if there are trees nearby or it is clay soil. Doing something smaller than is covered by Part A of the Building Regulations might need more careful engineering, and for that you would need to know soil conditions.
Natural materials are less well covered by the Building Regulations
Masonry builds are very well covered by the Building Regs. Timber frame for example is less well covered by the documents but often that is designed by the supplier who will have their own engineers.
It is more challenging to convince building control officers that materials like rammed earth, cob and straw bale have been designed right and won't fall down, because they are not so well defined structurally and it is difficult to put a figure on their strength.
Andrew recommends that if you're wanting to use one of the less common building materials it's good to get an engineer on board early so they have time to research, and ensure there aren't any difficulties that are likely to arise.
Steel or timber frame?
Steel is very stiff and an efficient material from an engineering point of view. Steel beams can be made to be near identical, and if you have limited head room it can be hidden effectively. It is readily available, fairly inexpensive and builders know what to do with it. With steel though you would normally need to get someone else to cut it for you and it has to be right first time.
One advantage of timber is that it can be cut on-site to get the exact size required. Every timber beam will be different so can have more of an aesthetic quality. And being a natural material it tends to have more movement, particularly from drying and shrinkage with green oak. Timber also benefits from less embodied energy than steel.
As a client, it's important to manage changes in the right way
Andrew uses the example of a current self build project he's working on, where the scheme is basically done but the client is very hands-on, has done a lot of research and is very open to making changes. Andrew says that it is refreshing to work with a client like that who, because of their research, is able to understand the more technical details and can have more informed decisions about whether to make changes to improve the scheme. He does add however that there's a fine line between that and an interfering client who keeps changing things!
“Because when you’re trying to move things along quickly, change needs to be managed as a process. There’s nothing wracks up bills more quickly than constant changes by the client because it can often involve going right back to square one and re-doing a whole load of stuff, which isn’t readily apparent to someone who doesn’t understand.
“So it is important to manage changes in the right way as a client. Which doesn’t mean to say that you can’t change things but you need to understand there may well be consequences.”
Use personal recommendation to find a good structural engineer
Andrew recommends that this is the best way to find a good structural engineer. Ask a previous client about whether they produced things to a reasonable time scale, were the costs reasonable, and even just getting a gut feeling about them from speaking on the phone.
“Do they feel a bit stand-offish, are they someone you could have a discussion with? Are they the sort of person that you would want working for you or not? Because sometimes if people don’t speak to you nicely on the phone the first time, how are they going to talk to you later on?”