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The Conceptual Structural Design—On The Engineer Trying to Make Sense of Art
After sorting things out with your client or team—the very first task of a structural engineer is to make physical sense of what the artist, architect or owner want. This is not the easiest task in the world as it involves the desire of many individuals, potential profit, usability, competition or simply ego. Engineers, especially the ones dealing with architects and developers on a daily basis understand what I’m saying here. This is the synergy we usually try to share when we are trying to figure out whether a nice concrete or steel frame layout that we know, is going to work. However, this is not always the case.
I’m not an architect, so I’m not claiming any expertise in that field and neither is my objective to disqualify such an important role in a building project. What I do know though; is that in the very preliminary stages of a project, ideal use of the space is normally defined by our architect friends. Their visions are not necessarily easy to accomplish based on the laws of physics. Even if a very clever or aggressive engineering firm has made an idea happen at point A on the globe doesn’t mean that it’s going to work or be easy to make it work on point B of the same planet. Engineers—in their search for showing off their knowledge, are usually deciding to solve every single problem by adopting complex math or physics… if required. Some others will advertise themselves as the ones trying to find the simplest solution or the cheapest one—normally this is the one desired by the owner or developers. What I know for a fact regardless of the desire or motto; is that the solution needs to follow simple physics IE the structure should be able to withstand the building mass times acceleration… which acceleration? That’s for another blog.
Anyways, the idea of conceptual design is to let the architect know what is required as the main structural system as a minimum. With defining a system, by this I mean defining the overall geometry of the structural components, such as columns, slabs, walls and beams of a project. The concept will obviously depend on the use of the building, the use of a specific floor or area, the location and soil condition, and local regulations. There are at this stage a number of variables that could affect the preliminary sizing of the layout. But this is a task for an experienced structural engineer who should be able to anticipate major changes along the road and to estimate conservatively; the sizing and location of structural components. This is easier to say than actually implement and could take a good deal of time for a team in an office to come up with an efficient layout or preliminary solution.
Over the years, I’ve seen crazy designs and according to engineers the solutions work. You can walk around your city or town and you can easily identify this craziness and at the same time you get the most typical structural engineering answer—”They work!” For gravity loads they totally do unless you are looking at a collapsed building or something like that. However (and here is where I reserve my opinion), have a cool building with such as an “outstanding” system (discontinuous columns, long spans, big openings, large spaces, etc.) doesn’t necessarily mean that it could work for a medium to high intensity earthquake. Most likely, the engineers have followed the proper code and made sure that his liability from the design point of view is not compromised. The question here now becomes; is the code the right document to follow to make sure these “outstanding” systems work?
I was presenting a case for a building that was designed using a performance-based seismic design approach. In one of the slides I was showing a view of the elevation of the building. Clearly, one or three of the columns there were discontinuous, this means that they were cut somewhere at a transfer level and followed by another vertical system to the foundation. After my presentation I got many questions regarding this new approach but one question kept me thinking and was voiced by an experienced structural engineer in Chile. My Chilean colleague was actually asking me if the cartoon or elevation had any typos or mistakse when I was showing the layout of the columns. He couldn’t find the vertical system going directly to the foundation! I explained how we normally deal with these changes in North America and this developed into a very interesting conversation. I was trying to explain and convince him that this system works, but he kept on asking me under what earthquake demand. This is when I started to worry—as everything we do to support our claim that something like this works is based on complex models and analyses using sophisticated math. I couldn’t find any explanation or even one example showing that those systems actually worked during and after an earthquake.
We have worked for many years on defining a proper methodology to get a solid initial system that not only works for typical and common loading conditions, but also for eventual load or events following not only code requirements but also current methods and experience. Our mix at PBRV is between technology development, earthquake engineering and experience with these sorts of events before, during and after an earthquake. This has allowed us to define structural systems that are capable of meeting clients or team members’ requirements and also add value for eventual types of loads in the future that are commonly above and beyond code regulations.