On Shopping Bags & Creepy Buildings

The advantage of squashing a facade

Until we thought better of it a weekly shop meant filling disposal plastic bags, provided by the supermarket, with our groceries. If you only had a few groceries to fetch and you decided to walk or if your car was parked a reasonable distance from the supermarket entrance then you may have noticed a curious property of plastic bags.

When they are initially filled they work rather well, however if the contents of the bag are reasonably heavy, for example  some drinks or a bottle of milk, then by the time you have reached your destination the bag handles have stretched. If the contents are very heavy, and the walk long enough, the handles may even have stretched to the point of breaking.

The interesting question is why this should be so? If the shopping bag performed satisfactorily when it was first picked up, why have the handles stretched by the time you get home? After all nothing additional has been added to the contents of the bag since you left the supermarket. The bag is carrying exactly the same weight as before. Why was it ok to begin with but not afterwards?

In engineering terms this would be described as increased strain at constant stress or in layman’s terms increased stretch without a corresponding increase in load. This is the definition of a phenomenon called creep. Creep happens when the internal structure of a material starts to become rearranged due to the effect of loading. Some materials, like plastic, are more prone to creep due to the nature of their internal structure, however all materials creep a bit under sustained load. It is worth noting that while our shopping bag example is based on stretching creep can also be a squashing effect.

A good example of creep that is more directly related to structural engineering would be the behaviour of old timber floors, which are often bowed in the middle. Another example would be the extension of bridge cables, which must be taken into account in their design. Intuitively it would seem materials progressively stretching or squashing over time is a bad thing. What would be interesting is an example where creep was actually a good thing.

In the late nineties I found such an example thanks to a rather demanding architect [that is not a bad trait in an architect]. He set the challenge of designing a building with a brick facade that was free from movement joints. He viewed movement joints as being ugly, a view with which I had some sympathy. If you haven’t noticed them before now, you will after this post and you will find them ugly too.

The building was shaped like a horse shoe, but with the open end enclosed by a full height glass wall. The perimeter of the horseshoe measured roughly 300 m. 

In case you are unfamiliar with common practise vertical movement joints are normally included in brickwork every 12 meters. You can therefore appreciate the nature of the challenge.

The solution to the problem was rather ingenious. I can say that because it wasn’t my solution. I was a young engineer at the time and still had much to learn. That said having a genius idea is only part of the answer and said genius usually still needs several less experienced, but enthusiastic, engineers to help him work out how to prove the solution will work. I was one of those lucky engineers.

Modern brickwork tends to consist of two thin skins with a cavity in between to keep water out. The two skins are tied together with wire ties. This is the archetypal cavity wall. For most buildings of any size the brick is supported on a floor by floor basis by the building structure and is therefore not load-bearing. The genius part in this case was to go old school and construct a reinforced concrete frame with thick load bearing walls. The concrete floors were supported on corbels embedded in the brickwork.

To understand why this was clever you need to know something about movement joints and several things about brick and concrete.

Movement joints are required because brick expands and contracts. Without relieving joints this will cause cracking. The greater the joint spacing the greater the movement. There are several reasons for brick movement. 

Firstly, bricks expand when wet and contract as they dry, however only part of the expansion is recoverable, as some moisture chemically reacts with the brick and some fills the open pores and will eventually evaporate. Secondly, bricks expand and contract due to temperature variation; they expand when warm and contract when cold. The shade, colour and type of brick affect the magnitude of this effect. 

Conversely concrete shrinks. It does so because the free water, which allows it to be poured, starts to evaporate as the concrete cures. This results in a reduction in volume that is manifest as shrinkage. One of the reasons concrete is reinforced is to control shrinkage and to prevent cracks from developing.

For our building combining the concrete with a soft brick and mortar was intended to pit brick expansion against concrete shrinkage. We worked out that concrete shrinkage could be directed via the corbels to clamp the bricks tight and prevent them from expanding due to irrecoverable moisture movement [the two biggest effects]. These actions are not instantaneous and could therefore be neutralised by creep.

This sounds simple now that it is written down, however at the time we were not sure that it would work. Many hours were spent researching, modelling and in the end testing our solution. In the end we could not make the whole wall work without joints, but the spacing was more than 100 meters.

So there you have it sometimes a creepy building is a good thing. Its never a good thing for shopping bags.