In the past few years; we have seen some spectacular collapses of buildings and bridges. This is quite inexplicable to today’s structural designer and engineer who puts enormous effort into the careful use of materials and huge safety margins. In addition, it is not only about care in design but also in putting into place monitoring mechanisms for the life of the structure.
However, when a designer is operating at the limit of their expertise; mistakes still occur. One of the questions with the Deepwater Horizon oil rig which exploded with massive damage was in the use of cement (coupled with nitrogen gas) a few kms under the seabed – as to whether it did indeed have the strength required to prevent a massive gas and oil surge. As we know, cement as a proven structural material has been around since Roman times – but for perhaps more predictable environments.
The typical approach a structural engineer follows when assessing materials for safe use and which would give some background to the building you live in, is as follows.
All materials used are extensively lab tested to determine their structural properties such as tension and compression under loading. The design strength actually used is considerably lower than these figures.
Design Strength Varies Depending on Materials
The calculation of design strength varies from concrete and steel which are fairly predictable and of uniform quality to wood which is rather varied. Wood is considerably more variable in strength as it could have an unusual number of knots or come from a diseased tree. Thus the safety margin has to be considerably higher.
For example, the Douglas fir, has a safety margin of 5.5 (versus 1.4 for Steel). For example, Douglas fir has a compression maximum strength of 51 MPa (7430 psi) and based on the safety margin of 5.5; a design strength of 9.3MPa (1350 psi).
However, (as we know from the Twin Towers disaster) steel can have problems for structural support. As you may recall it is weak in fires and must be protected in all buildings.
Additional Safety Margins
Structural engineers build in additional safety margins by overestimating the dead and live loads and selecting supports one size up from what the design suggests.
Beware when you are at the Limits of Technology
However, despite all this care; you have to be careful about operating at the limits of technology and as to whether you will see sudden loads well in excess of what you designed (DeepWater Horizon) or indeed, your materials exposed to conditions they were never designed for (Twin Towers).
Try and Anticipate What Happens in the Future
In Africa recently, there have been a few spectacular bridge collapses where it would appear that the corrosion of the reinforcing structures were a cause of a weakening of the structure. Coupled with theft of reinforcing steel. The last is hard to understand but this is what was reported.
In essence, what I am saying is that even with considerable planning and design expertise taking into account the relevant safety margins, all structures should be monitored throughout their lives. With today’s technology and of course internet connectivity, this is surely achievable without too much expense.
Thanks to 101 Things I learned in Engineering School by John Kuprenas with Matthew Frederick.
Andrew Heller makes an interesting point: Technology is like a fish. The longer it stays on the shelf, the less desirable it becomes.
Yours in engineering learning
Mackay’s Musings – 25th July’17 #662
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