I’ve been in a long-running conversation with anther engineer about grandfathering, and it occurred to me recently how hard it is to pin down changes in load with respect to physical changes in a building. In other words, there are a number of possible combinations of load changes and more possible alterations to a building, and it seems like it should be possible to have some correlation between the two. But I don’t think it is.
Before I try to explain the vague statement above, a moment of definition. I’m only going to discuss dead load (the weight of the building), live load (the weight of the buildings occupants and their stuff), wind load, and seismic load. Note that these are not entirely independent: seismic load is based mostly on the dead load, and buildings with high live load tend to have high dead load as well.
The photo above is the Biltmore Hotel, right about the time it was completed in 1913 across the street from Grand Central Terminal. I’m going to use it for my examples of increasing load. A few possibilities:
- You could add a few floors (two, five, ten…for the purpose of this discussion, it doesn’t matter) to the Biltmore’s 20. This would increase the dead load (the building would get heavier), the live load (there would be more hotel rooms with people and furniture), the wind load (it would be taller, so it would catch more wind), and the seismic load (more weight, higher up).
- You could keep the building physically as is, but change it from a hotel to office space. This would increase the live load and keep everything else the same.
- You could keep it physically as is except for installing terrazzo floors everywhere, getting rid of wood flooring in the rooms. This would increase the dead load and the seismic load, but not the others.
- You could add a set of huge billboards on the roof, which would increase the wind load a lot, and the dead and seismic loads a small amount.
- You could fill in the deep light court, increasing the dead, live, and seismic loads but not the wind load.
- You could strip the heavy masonry curtain wall off and replace it with a light-weight modern wall, and add a story on the roof. The total dead load and seismic loads would decrease, and the live load and wind load would increase.
- You could strip the heavy masonry curtain wall off and replace it with a light-weight modern wall, and fill in the light court. In theory, if you did it right, all of the loads would stay the same or decrease slightly.
I could keep dreaming up scenarios all day, but I think I’ve made the point. There are all sorts of ways to modify the building that will increase some loads but not others. (If the last two seem far-fetched, this is probably the time to point out that in the 1980s, the last option actually took place.) There is no obvious correlation between “expansion” and load increases.