Quote:
Originally Posted by CGII
When one designs a building to withstand a disaster, one designs the building to stand only long enough to safely evacuate the occupants.
Basically, a material property of steel what is called a 'modulus of elasticity.'
What this means is that a steel member will have a certain strength when it is rolled out of the mill and installed in the building (notated as Oe on the graph). However, when great force stresses the member (say from an earthquake or an airplane impact), the member deflects and actually becomes stronger (notated as Ob on the graph). When the steel deflects further than that point, failure is iminent. This means that once steel has deflected, it may still stand but it cannot be deflected again.
Building safety design accounts for the standard strength of the steel, expecting that the modulus of elasticity of the steel will meet expected disaster loads forced on the building BUT with the understanding that once the building has survived the disaster, the steel is effectively damaged and no longer safe for continued occupancy. The building will likely need to be demolished. |
That's essentially the point in my post on the last page. The key is to design and construct the building to allow the building occupants to safely evacuate in a reasonable amount of time. Afterwards, the building (especially if it's hit by a jetliner flying at 300+ mph) will likely be too damaged to be considered safe for occupancy and to the surrounding environment, so it needs to be taken down.
And to expand on the graph you posted for those who want to learn a bit. This is called a stress(σ)-strain(Ɛ) curve.
- Stress is calculated as the force (F) on a material divided by the cross-section area (A) of the material (F/A). Essentially, it's the amount of force a material experiences.
- Strain is the amount a material stretches or compresses (I) divided by the original length (L) of the material (I/L). Essentially, it's how much a material can deform.
- The modulus of elasticity (E) is the slope of the stress-strain curve up to the elasticity limit (e).
The key area to steel's stress-strain curve (which is what's shown) is the section up to the intersection of σe and Ɛe, which is the elastic limit. A steel section can be stretched up this point, and it will go back to its original length once the force is released. For the most part, you want to design your steel members so that they don't encounter forces above the elastic limit, since you want all the members to remain the same length. In all structural design, you need to design for a certain factor of safety above the typical maximum loads, so you choose the member sizes accordingly (size means both the length and the cross-sectional area).
Quote:
Originally Posted by Kanto
I never issued such a claim. I only said that I know that building a skyscraper at that site is a complex process. I never said that I know the details of the building's structure. One reason for my topic is to find out if there are any technical or economical problems with my idea. If you know any details that would affect my idea, please visit my topic and post them there  |
Actually, I quoted what you wrote word-for-word. And part of understanding "the complexity of the site" is knowing, at the very least, the basic structure of the building. Actually, you should understand a lot more than the basics if you're trying to push for your favored design. That way, you can make rebuttals and criticisms based on informed opinion.
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