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.
However, last night when I was watching the causes of the BP Deepwater Horizon catastrophe (with my fascinated non-technical wife, I might add), I realized that when a designer is operating at the limit of their expertise; mistakes still occur. One of the questions with the Deepwater Horizon 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.
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).
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,