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I’d like to share with you a little bit about the design of the Cherish Primary School, as we are utilising an interesting structural system . This system has not been used by EMI Uganda before,  but we hope that it can be the direction in which we will continue to develop as an office.

Firstly, a bit of background. The majority of buildings that EMI Uganda designs are relatively small, single-story construction. We aim, as much as possible, for simplicity in building design. We believe this provides the most appropriate structure at the end of the project, which will serve our partner ministry’s needs most effectively. This is a topic which I will expand on in further posts.

Taking local residential construction practices as our baseline, this has historically used village-brick masonry and timber trusses. Both are relatively low-skill materials to work with. An upgrade to this design is to use steel for the trusses, but the cost is greater than a site-manufactured timber truss. (Steel also requires slightly more specialised design and fabrication facilities.) Using this method, the masonry walls transfer any lateral (horizontal) loads into the foundation along the length of the wall.

Normally, the main source of these lateral loads is the wind blowing on the side of the building, however, we do also need to consider a small chance of lateral loading from seismic sources.

As the buildings get bigger, it becomes inappropriate to use masonry walls in the same way. They have a maximum slenderness at which they can work and, eventually, become too skinny to support the loads that they are required to. The wall becomes unable to transfer the lateral loads into the foundation. Ignorance of this fact is one major way in which inappropriate and unsafe buildings can be constructed.

It is at this point that another framing system is required. Usually, this is where concrete, the granddaddy of construction materials, steps up to the plate.

Reinforced concrete is a great material, and provides a stiff moment frame that is good at resisting lateral loads from either wind or seismic sources. It is a good framing solution for medium-sized buildings, and particularly good as we start to consider multi-storey structures. The lateral forces are resisted in the stiff joints where the beams and columns meet. Any lateral loads are dealt with within the concrete itself and transferred safely to the foundations. Masonry is still used as infill panels to define the walls in the structure, and the masonry is usually constructed after the concrete frame is in place. Structurally, the masonry is doing nothing other than looking after itself, it is contributing nothing to the overall stability system of the building.

However, this approach may seem a bit wasteful. “The masonry is there!” you cry, “Shall we not use it?!”

One way of obtaining a best-of-both-worlds approach is to construct the concrete frame as before, but design it in such a way as to actually rely on the masonry infill wall for stability. The wall is then built inside the concrete frame and tied into the frame so that they act together. The concrete structure uses the masonry for help in providing lateral support, and the masonry benefits from the ductility (bendy-ness) of the concrete.

NOTE: Another way to achieve this without the need for formwork is through confined masonry. The masonry walls are constructed first, and the concrete frame is cast around the walls. This usually, however, results in a messier-looking building that requires some additional finishing work to make it presentable.

Something that we are exploring in EMI is to see if there is a way to move beyond concrete buildings for our projects. Concrete, as I said, is the granddaddy of construction materials, but it is not without its limitations.

The placement of reinforcement and the preparation required to set formwork takes up construction time. Concrete itself can be quite variable if it is not mixed or placed correctly. And it takes a few days before the concrete has cured (set) sufficiently to allow it to be strong enough to strip the formwork and carry on with  construction.

Steel, although it is more expensive weight-for-weight, requires less material to do the same job as concrete. It can also be erected faster and construction can continue immediately following erection. EMI had previously run a cost comparison and found that, in materials alone, an equivalent classroom building constructed of masonry and steel is likely cheaper than one constructed of masonry and concrete. Elements of the building can also be standardised and manufactured offsite at EMI’s Workshop. This contributes to the savings that can be made in construction time.

Steel is much more flexible than a concrete frame, and diagonal bracing is usually used to provide stiffness. The use of diagonal bracing is more efficient than trying to construct a steel moment frame at this scale. It’s easier to form moment frames in concrete, rather than steel. However, if we’re planning to put masonry walls in place, they have to be coordinated to make sure that they don’t clash with the bracing.

But maybe there’s another way.

“The masonry is there!” you cry, “Shall we not use it?!”

Can we use the masonry walls that are there to hold up the rest of our steel building? Can we take the principles from the concrete-masonry hybrid and produce a steel-masonry hybrid building?

We believe that it’s possible to take similar analysis methods used for the concrete-masonry building and adapt them to successfully analyse the steel-masonry building. That way:

• The structure is more efficient, using more of the materials that are already there.
• The timescale on site is less, because the steel elements can be prefabricated offsite and the construction can continue directly following the erection of the steelwork.
• The cost is reduced, making better use of the client’s resources.

We are not aware of many buildings constructed in this manner in Uganda, but we’re excited to start developing this system and to see if it can be implemented for even more of our buildings in the future.

In order to design this system properly, we need more of an accurate idea of the strength of the materials, particularly the bricks. Stating that, “It’s probably fine,” accompanied by a Gallic shrug, is not going to cut it any more.

We carried out some brick tests this week, and I’ll go into more depth on that topic in the next post.

NOTE: To my fellow structural engineers who are reading this, I am aware that some of the force arrows are drawn in the wrong direction. I see the arrows as a clearer indication of load path, rather than directly representing tension/compression. Let’s call it artistic licence. If you still feel the need to complain, I admire your pedantry and award you one internet point.