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.

As I enjoy my morning coffee and get ready for work with a great view of the bay— I notice that the hanging lamp is swinging unusually and the cladding or exterior windows start to make a cracking noise and it’s getting louder.  I put the mug aside on a table and the coffee spills onto the carpet, the floor starts moving faster and now I can barely walk to the kitchen to get something to clean up the spill.  Now, I can see that some windows are starting to crack.. for real life!

My TV, bookshelves, kitchen wares, glasses are all on the floor and I can hear many people screaming in the other units of the building.  I try to remain calm and get some cover under my table but now the movement is slowing down and it seems everything is going back to normal.  After some hours, I get the news that we have just had a moderate Mw 5 earthquake and some news anchors are reporting some minor damage in town with no casualties.  After a day we find out that 4 people have died of heart attacks and one by exiting too far from a building and falling abruptly on his head.  There are some reports of minor damage to roads, a couple of landslides but everything seems to be under control with minimum downtime.

Now—aren’t we supposed to experience a “Big one”? Since I know a bit more than the average person about these types of things, I’m kind of wondering what would have happened if an earthquake with the magnitude that we are likely expecting had struck—say a magnitude of around 6.5 to 7.0.  A lot was going on in my apartment, and it was only a 5!  When I mean with “a lot” I mean a lot of non-structural damage which are little things in the overall scheme of things, but my forecast for “the Big One” is not a comforting one. 

Ok Ok..I was just making this up, but I wanted to make a point here and I’ll make it from a structural engineering point of view.  When we are working on the structural design of a building and we are trying to come up with a reliable system to withstand the actions of earthquakes, we follow the most current building code.  It is widely accepted and specified in codes that any design should follow basic energy conservation principles.  In this case, the energy released by an earthquake near the building should match the energy dissipated by the building.  Now let’s see what we understand by “energy”.  In terms of the earthquake energy; we commonly refer to the amount of movement that the earthquake releases through local movement. We can also add noise but mainly ground movement (shaking).  When it comes to the energy dissipated by the building, we are referring to any building movement or interaction between the building and the bearing soil, or between the building movement and its content. There is noise as well but let’s focus on the movement.  Most of the time the energy coming from building movement or from the interaction between the structure and the content or bearing soil is capable of matching the energy coming from the ground movement.  But sometimes (for those major earthquakes that occur once in a while), the movement of the building and the interaction between the building and other elements is not enough to match the earthquake energy. 

Engineers have been, over the years trying to come up with ideas on how to add more dissipation mechanisms to buildings or to further understand the movement of buildings and the interaction between the structure and its content and bearing soil.  In an ideal scenario, where resources are infinite or abundant and profit is not part of the equation; we can design buildings to match any input energy with the building movement using what we understand about the interaction of the structure with its content and soil.  In a real scenario, however, this is not always an option and we need to give up something in exchange.  The subtraction or difference comes in the form of either an additional solution or system that is capable of dissipating the differential energy. The other option is that we simply accept a level of damage to the building and dissipate the energy through what engineers usually call “inelastic energy”.  Inelastic energy is a nice term, but for the regular Joe it simply means tons of damage. This damage can include large cracks in some walls, cracks or destruction of partition walls, broken glass and windows, heavy furniture falling apart, door access issues, etc.  Overall this is a real mess with (hopefully) a clear exit from the building after the shaking.  This is what we normally refer to in codes as a life-safety standard. 

Our previous blog was titled: Life-safety standards defined in codes. we explained the concept of how earthquake energy is dissipated through damage. This isn’t the nicest thing to know about when you are probably spending a lot of money on that new condo. Unfortunately for you, these types of earthquakes are expected to occur once in a while. This becomes a problem for us though if one happens in our life time… I mean like right now. The obvious question then becomes “What should I do”?

There are many things that you can probably do to prepare ahead of time, such as putting together an earthquake kit, setting up an evacuation plan and gathering point afterwards, thinking of another city to live in (just in case), keeping contact information handy, retrofitting your house, buying insurance (and knowing what you are buying), etc.

No matter what your plan is, the one thing that you’ll experience for sure and that you need to know is where your safe zones are. You want to live, I guess, after all that mess. We have also explained before where are the structural systems that are in charge to withstand the earthquake shaking and dissipate energy (in case). In a building, that safe area will be most if the time in the middle of the building where the main exit points are located. In architectural terms is called the stair or elevator shaft, in structural engineering term is called the building core. The walls around that core are called the core walls. On top of each exit point, where you either access the stairs or the elevators, are located some beams that are designed to experience large deformations and observed damage during an earthquake. Some engineers called those beams as the weak links, and they certainly are in terms of strength, but they are not in terms of sustaining earthquake demands or movement. I called them the vital links.

Certainly staying below those elements (weak or vital) is not such as a good idea, but staying very close to that core is certainly a very good move. Our way of seeing the actual movement of people is a trend to escape quickly from the building towards the stairs. Our recommendation is to go to your main door, open it as soon as you can, and wait there for 30 to 40 seconds. Then go down the stairs and exit the building. Our additional suggestion and since you’ll be likely holding that door is to find your choose, because you’ll need them – many debris and glass on the floor is not such as a good idea for a barefoot walk. Take any other item you may need. I personally keep my emergency backpack right beside the door and next to where I put my shoes.

Now, why do structural engineers design a single core and why in the middle? Well, many birds can go down with a single stone. First, the idea of having all exit areas in a common area makes kind of sense from an emergency point of view. Stairs, however, have their own regulations and more than 2 exit points must be assigned to each floor. However, both of them belong to the same core and to the same seismic event resisting system. Second, a core contains walls and beams, being the beams at top levels the one in charge to dissipate the building energy mostly. With a lumped damage area, the buildings are easy to repair without affecting the units themselves. Third, the idea of a single core and having one system to take care of the earthquake allows owners to have more living space. From the construction point of view, there are some important savings and the industry has a good handle of that process already. There are several other alternatives from the technical point of view, but mainly the ones I pointed out here are relevant to my safety points. As the building tries to dance under the earthquake moves, some building or unit spots are important to avoid. Do not stay too close to the exterior part or balconies.

We design buildings under earthquake loads to limit the maximum movements to be at the very end (exterior) of a plan or floor layout. Stay away from large lamps or heavy furniture. Depending on the floor you are, those elements could become a hazard. In these cases, the best option is to adopt the drop, cover and hold approach, but no guarantee that you’ll be injury-free. Our best approach is to keep your main entrance clean of heavy objects and run there as soon as you are start feeling the shaking. In case your unit has some gas appliances, you need to turn that off right away. Those are the number one candidate for igniting a fire after the earthquakes. You may want to keep yourself both injury-free and dry (sprinklers will go off). Now that you survive…you can plan your building exit. You may see concrete on the floor, especially near the exit points. If that’s the case, then the “weak” links worked and the building performed as plan, or at least to at a certain seismic design performance. Some wall cracks could be also observed, ideally the cracks should be kind of horizontal and at the end of the walls near the door or exit areas. If you see large diagonal cracks in the middle of wall, you should really exit that building ASAP! That’s a bad sign and most likely the building didn’t performed as planned or at what codes and practice dictate. In technical terms, a horizontal crack at the end of a wall is also part of the energy dissipation mechanism of a building and we call it a flexural type of failure. That type of failure is not really a failure and is mostly a good behaviour. In seismic terms we call that as ductile behaviour and are similar to stretching out a gum – endless deformation without failure. The diagonal crack in a wall is the one we tried to avoid and in technical terms is called a shear crack and is associated to a sudden type of failure. In other words, a diagonal crack is a sign of a potential failure and is a good idea to run – hopefully you don’t see that.