HPH329 : What does a structural engineer do? – with Beth Williams
Beth Williams from Build Collective outlines the role of a structural engineer and explains what tasks might be undertaken on a house build.
Interview with Beth Williams
Beth had wanted to be an architect since she was a teenager, and graduated from university with a degree in architecture and structural engineering. However, at the time she graduated, the UK was in the grip of a recession and the only job opportunities she found were in structural engineering. Part of the reason she has since gone on to work in the low-energy space, is that she enjoys the close working relationship with the design team.
Beth is a Chartered Civil Engineer and Certified Passivhaus Designer. She is an associate at Build Collective; a bespoke engineering consultancy in Bristol.
Structural engineering is a sub-discipline of civil engineering
Civil engineering sits at the top of the hierarchy. It covers things like highways and drainage.
Structural engineering is a specialism within that, but can still be quite broad. Structures can mean a focus on things like bridges and masts, or it can be for buildings.
So if you're looking for a structural engineer for your project, you can either opt for a chartered civil engineer (with a specialism in structures), or a chartered structural engineer. Beth explains that there isn't really any difference in terms of insurances and liabilities. The qualifications to look for are: the MICE (Chartered Member of the Institute of Civil Engineers), and the MIStructE (Chartered Member of the Institute of Structural Engineers).
They work as designers, under the principal designer
If there is no architect on the project, such as for a small domestic knock-through, then the structural engineer may take on the required role of principal designer looking after health and safety.
For most other projects though, the architect is the principal designer. They coordinate the flows of information between the design team members, and produce a pre-construction pack. That is all of the information from all of the designers, and all the health and safety risks. This is then passed on to the principal contractor to take as their responsibility once the construction phase gets underway.
Small projects might not need a structural engineer
If you are redecorating or moving an internal wall that isn't structural then you probably don't need a structural engineer.
Part A of the Building Regulations gives a generic outline of the structural principles, which could be used by an architect to design a very limited building such as a garage or an extremely basic house.
It used to be the case that people doing architectural work would have cover in their professional indemnity insurance for designing small structures. However, since Grenfell and a number of basement collapses in London, a lot of them have lost those insurances so the work has come back to the structural engineers.
Domestic clients will usually pay a fixed fee
Larger projects tend to be costed up on a percentage basis, whereas domestic clients are likely to pay a fixed fee for the defined scope of works. Beth's advice is to always check what is on the scope of works, and the number of passes included.
A lot of engineers base their fee on responding to a set of drawings once. Build Collective however, do a two-stage pass. This involves some rough sizing and layouts as a preliminary design. The architect uses that to develop their details before returning to the structural engineer for their detailed design. Having that extra pass allows for tweaks to be made based on new knowledge that might be acquired during the process, such as ground conditions for foundations.
Many factors impact on the structural system
Corner windows, fancy overhanging roofs, and flat roofs are just a few elements that will impact the structural system. This all feeds into the “Have I got enough wall?” question, which is for the structural engineer to answer. Things like large glazing and an open plan design will all obviously limit the amount of wall that they have. So it's about how much wall there is to make the structure stand up, and evenly distributing it throughout the building. Large stair openings and double-height voids mean there is less floor area to be able to distribute the loads.
Where there aren't walls, it might mean putting in portal frames or goalpost frames to give stability.
It's not just the vertical loads that need to be considered either. They also need to factor in whether the walls can withstand the horizontal load when wind is blowing on them.
Local structural engineers may have better knowledge of ground conditions
One advantage of hiring a local structural engineer is that they will be familiar with the area and its challenges.
For example Beth says that there are many cheap building plots in Bath because the ground conditions are often horrendous. It has a lot of historic landslip material that is volatile to work on. And Bristol has lots of coal mining areas and slope stability issues that will prove challenging.
Beth suggests a good use of money would be to pay a structural engineer for a few hours to look at geographical maps for your site and provide advice.
She would also recommend having a proper geotechnical survey for a new build, which costs around £3,500-£5,000. This involves a specialist geotechnical engineer doing bore holes or trial pits. They will log the findings and send samples away for lab testing, and report on the different structural and chemical properties of the ground.
If you can get a good ground investigation early on, you mitigate a lot of potential risks that could occur later on.
Your foundations will depend on your ground conditions
There is a range of sizes of ground particles, from large rocks down to gravels, sands, and clays.
- Rock – For rock at depth, you might be able to pile down onto it, and lock onto the top surfaces
- Gravel – Can be very flexible and have good bearing capacity
- Sand – These can be good because they don't have the same volume change potential you get in clays
- Clay – Clay heave can be an issue. The clay will swell and shrink over the seasons as it takes on more or less water. To get around that, you dig deeper to where the clay isn't affected by the heave. That may then be too deep to dig safely, so would need piling
Concrete may be something that you're keen to avoid but it does have its place.
Beth uses a lot of insulated raft slabs on low energy projects. It's a good solution for timber frame projects because you need some weight in the building to hold it down. If you are going to use concrete the important thing is to use it efficiently.
Another foundation option could be screw piles. They have some tension capacity in order to do the uplift for a timber frame. However, they're not going to be possible in every scenario, and Beth warns against clients fixating on them before knowing whether their ground conditions will be suitable.
(AECB members can access a webinar recording of Beth discussing alternatives to concrete foundations.)
The design starts with the roof
Beth uses a top-down approach, starting with the roof design. This is followed by any suspended floors, then the walls and lintels in the walls.
Even before she does these vertical load takedowns, she will look at overall stability. This incorporates the wind and snow loads.
A surprising level of information is needed to do the structural design
There are lots of things that feed into the structural design that may not be immediately obvious.
Staircases are a good example. Where there's a stair opening going up through the floors there needs to be a stair trimmer. There may also be a balustrade to prevent falls. What that balustrade is made out of will impact the structure. Glass obviously has more significant limitations on the deflection it can take before it cracks, than a lot of other materials. It has different fixing requirements too. Another consideration is how the balustrades for the stair itself join onto the rest of the balustrade.
When clients arrive with a specific product that they want to use, Beth sources all the data from the supplier in order to do the design. She has developed her foreign language skills because often the technical information isn't supplied in English!
Don't be afraid to involve an engineer early on!
This is especially the case if there is a specific product or system that you want to use. Get advice early to see how that might impact what the structure looks like, where the windows might go, and how many walls you might need inside. Beth would then do preliminary rough structural layout markups, ahead of planning, to show where the beams and frames could go. The architect can feed this back into their designs.
Find out more
Visit the website of Build Collective
Follow Beth Williams on Twitter
Calling future engineers
Water expert Cath Hassell has featured on the podcast several times before (discussing topics such as saving water, rainwater harvesting and dealing with sewage). However she is also the creator of the Frankie the Flamingo series of books for children. So if you have a budding engineer on your hands perhaps The Mysterious Case of the Elephant that Forgot is a good place to start.
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