Loads: 4b. Seismic Loads Images

Okay, in this video I'd like to show some images about seismic forces and seismic measures taken to strengthen a building against earthquakes. So the code, the seismic code says in a mild earthquake there should not be damage to the primary structural system or to the secondary systems, the pipes, the ducts, the ceiling tiles, etc. Those are secondary systems. The code goes on to say in a moderate earthquake, there should be no damage to the structure, but it's expected to have some minor damage to the secondary systems. And in a major earthquake, there could be damage to the structure and to the secondary systems, but collapse is not acceptable. So here we have a building that collapsed. The seismic code is involved with life safety. It does not address damage to the buildings as much. Versus wind codes based on Miami-Dade says that you have to make sure you secure life safety. And also, you board up your windows, you do whatever else you can to reduce the damage to the structure. In an earthquake, there's no time, there's no warning, so damage is not addressed in the coat. Very good. So, let's look at this seismic retrofit of a building in California. First of all, this is an illustration of a soft story. So what are my options for lateral support, for lateral resistance? I have a moment frame, I have an eccentrically braced frame, I have a concentrically braced frame, and finally a sheer wall. All of these are addressed in a previous video. So the shear wall is more often than not the solution for wood frame construction. Now the bottom floor is the soft story and if we look at things it looks like we need to address north south and east west. Cannot address only one direction it'll fail in the other direction. So I'm looking at this direction here. If the earthquake were to come in this direction, then it looks like we have these braces between the cars. Okay, so we're covered there. Now, if the earthquake comes this way, all your columns are going to go with the earthquake, if there were not that moment frame there, they would all go with the earthquake, and then this mass would collapse the columns, and that's what we described earlier as the P delta effect. There was a lateral load, and it deltaed, or it drifted, and then the mass crushed the columns underneath. Very good. So let's see what we can say about this one. I need to, there is no eraser in this app, so I'm going to have to undo, undo, undo, whatever. Okay, so now if the earthquake comes this way and we had post and beam only, all of the columns go out of plumb and then the mass crushes and we have the P delta effect. However, if I use a concentrically braced frame, I'm going to lose one parking space. If I use a sheer wall, I'm going to use a parking space. So let's not use either of these. Instead, let's do a moment frame. A moment frame allows me to save the one parking spot, and it is made of rigid connections. Right here, the horizontal and the two verticals are rigidly attached to each other. And I don't have enough detail as to the attachment at the base, but I assume it will be made rigid. So this is a rigid moment frame made of steel. And when the earthquake comes, for example, from right to left, the column is going to do that. That's what it's going to do because it's rigidly attached to the base. So it'll move a little bit, but then it'll start bending. Moment frame bends. Bending gives me ductility. The horizontal, on the other hand, is going to try to maintain that 90-degreeness. Oops. Did I lose all my markings? Yes. So when the earthquake comes this way, the column is going to do that, and then the beam is going to bend accordingly. So now the beam is bending, the columns are bending. This is ductility. When the earthquake reverses, the columns do the opposite. So a moment frame, this is a sheer wall above me, and the columns are making up a soft story. So in this case, the material overhead is wood frame that's not very strong, unlike concrete or masonry. This one is going to end up being a weak story versus a soft story. Okay, so what else do we need to say about this? I think that's enough. Let's look at a weak story, also known as a discontinuous shear wall. This is one of the most difficult situations to engineer in seismic design because you have a very strong shear wall sitting on toothpicks and then the foundation underneath. Earthquake comes in and does that. The mass wants to go the opposite way and all of a sudden these toothpicks are broken. There is no remedy to this one almost. it's almost certain to collapse. Please note that this picture is not in a seismic zone. I'm cheating a little bit. But there is a shear wall here. There is another shear wall on the other side. And in between, there's reinforced concrete, which gives me a moment frame in this direction. And loads in this direction are resisted by the shear wall. But then there is a weak story right here, a weak story. Very good. Let me change this a little bit and let me take it from being a strong shear wall and a discontinuous shear wall at its base. Let me make it a soft story instead of a weak story. Well then, if you have major openings here in this wall, it is no longer as strong. So now all of a sudden this might end up being a soft story because what's above it is not that strong. Okay. The difference between weak story and soft story, weak is about the strength and failing versus a soft story is more about stiffness and movement. It moves a lot and then it does P delta. Okay, let's see what's next. We have a reduced beam section or a dog bone, and this one, what it does is it shaves the wide flange, top and bottom. What are we shaving? We're shaving the flanges. Where are we? We're at the support. At the support, shear is critical, not bending moment. The web is doing all the shear with all these bolts. That's who's in charge of shear. The top flange is in compression. The bottom flange is in tension. Together they're doing bending, but at the support the bending is minimal so you can take out some area. So what you just did is you weakened the beam at this location. You made it a little bit weaker and you are hoping that failure will occur here and that it will be a ductile failure rather than at the support, at the bolts, because that failure will cause collapse. Please note that the dog bone or the reduced beam section has to be provided in two perpendicular directions. So this one was tested in the 1989 Loma Prieta earthquake in California, and its performance was not that good. So it's been abandoned a little bit, but it's important to understand the logic of how it works. You weaken the flanges at the support. They're not doing much. You gave the earthquake a place to attack rather than where it wants to attack. So it's like perforating paper and you tear along the perforations. That's where the tear will happen. Or like a controlled joint in concrete, whatever. Okay. So looking at this sparking deck from that same earthquake, we see that there is a gradual failure instead of sudden. Because this one is a special moment-resisting frame. Special means detailed for ductility. There's rebar in certain places that makes it more ductile and less brittle. therefore, takes much longer to collapse. So this is an example of a special moment-resisting frame. And in the lecture earlier, I discussed the spectral response coefficient, r. And we said this is with the spinners on the previous slide. Sorry, just let it go. Anyway, this is an r value of 8 because it's special. detail for ductility. It's made of steel, very good ductility. Moment frame, very good ductility compared to sheer wall. So in those spinners and the slot machine, that's what, this one gets high marks. Although it collapsed, it just took time, life safety. Very good. Looking at this building in Hayward in California, this is City Hall. Therefore, the contents are extremely important and the structure is less important. The contents, the birth certificates, wedding certificates, death certificates, building permits, everything is in this building and therefore the secondary contents are much more important than the primary. Looking at this building, I'm seeing a brace here and I'm hoping that there are braces, I see it, there are braces in the entire bay, Otherwise, it would be a discontinuity with bracing skipping a bay or something. I am looking in the windows here, and I'm sorry I can't zoom, but I see another one here. Okay, and I'm hoping that all four levels are braced. So typically, typically, you want 25% of your bays to be laterally enabled, which means either moment frame or brace frame or eccentrically brace frame or concentrically brace or a shear wall. So a quarter of the bays, it would be nice, that's just a rule of thumb, to have lateral stability in a quarter of the bays. You have 20 bays, a quarter of that. Let's brace or moment frame or sheer wall, five of them. Very good. So here I have five bays, five bays divided by four, a quarter of them, one and a quarter. Okay, so they've braced two bays. Very good. Now let's look for discontinuities. It looks like this floor, the lower floor, the ground floor is taller than the next floor. That's a discontinuity. Looks like there's a balcony or a condition here, a setback. Okay, that's another discontinuity. Looks like there's a re-entrant corner here. That's a discontinuity. Looks like there's something really ugly in the front. That's a discontinuity. So it's not unusual to have discontinuities. And what that does is it increases the percentage of the dead load that we need to account for as base shear. Very good. So I went in the basement of this building because some of my students told me you need to go there. There's some very good examples of seismic resistance. So I went in there and down in the basement, it was awesome. Let's remember what we had, a four-story brace frame sitting on a concrete podium right here. And the superstructure is above here and there's parking down here. And at every beam column intersection, there is a transition to the foundation. And then there's these blue things, which are base isolators. And also there's these black things, which are fluid viscous dampers, which is going to be the topic of this lecture. So bear with me. So we see them here, base isolators and fluid viscous dampers. So let's remember that the contents of this building are extremely important, and we don't want a pipe coming down and flooding and ruining the documents. So when an earthquake starts doing its thing down here, let's say it's a magnitude 7 earthquake, it's not going to travel up as violent because of the base isolation and the fluid viscous dampers. So it'll be dampened before it makes it to the upper floors. So looking at this, we have the base isolator. And what it is, it's a layer, sorry, it's several layers of rubber and stainless steel. That's what's inside that blue box. And in the middle, there is a core, an empty core that has lead in it. Now, as the earthquake starts rocking left and right, rocking the building, this base isolator takes that energy of the earthquake and transfers it to heat. So it heats up the lead plug that is here. So the movement is quite significant in these base isolators. It could be one foot, two foot, three feet. So you can't do it in urban retrofit or something like that if you have neighbors because it's going to move violently. So sidewalks, landscape, et cetera, all that is going to be destroyed, but the building is going to be saved. Very good. Let's look at this animation here. I think it's a very good one. The building on the left has no base isolation, whereas the building on the right does. So as the ground moves, the shock absorber is a base isolator. So it's like a shock absorber. It's just layers of stainless steel and the liquid is barely moving. So base isolation. Very good. Here they are under installation on new construction. They are, by the way, used as seismic retrofits. If you have a brick building, a concrete building, or something like that, that is somewhat of a low rise. I don't know, four floors, five floors, they can jack it up and put these base isolators underneath the existing building. But the picture on the left is new construction. So let's talk a little bit about these fluid viscous dampers. And let's remember the superstructure is overhead. The garage is down here and the earthquake is going to come in here. Some of that shock is going to go into the base isolators. But then the rest, this part is rigidly attached to the lower level, and this part is rigidly attached to the upper level. In between them, there's a piston with fluid, and it's hydraulic. So as the ground moves this way, that piston expands. When it moves to the right, then the piston compresses. Expand, compress. Expand, compress the fluid in there. So that's how the fluid viscous damper works. And of course, here is tailored devices, the makers of these things. And of course, they're trying to make them a little bit more beautiful by having them displayed by female models. Okay, so let's talk about resonance. We talked about it in the lecture, and I would like to articulate that. We said resonance is critical if the period of the soil is similar to the period of the structure. Then you get resonance. Resonance causes amplification of damage. The damage will be much worse. So I'm not sure why this building did not collapse. It's a canopy at a gas station. It's a cantilever. And its period resonated with the period of the wind. not necessarily earthquake, and that thing was shaking like crazy. Another example that you're probably familiar with is the Tacoma Narrows Bridge of 1940. And this one, the period of the bridge and the period of the wind resonated. It was just 35 miles an hour, but this is a suspension bridge, and it resonated with the period of the wind. and it collapsed into the Puget Sound. Very good. Now, I'd like to look at this animation on the left from Professor John Lerr out of Cal Poly. He has four blocks. The blocks are identical. Their area, their material, therefore their weight is the same. But I can't write on here. Okay, but the tallest one, the red block on the far right, has the longest period because it's tallest versus block B in yellow is the shortest. And he's going to start, he has them on a roller, and he's going to start shaking the ground at different periods and see which ones resonate with which block. So let me animate it. Here, he starts shaking rapidly, therefore a short period. And block B has a short period, and it resonates, and you can expect the damage on this one to be very bad compared to the other three blocks. Versus a slow period, building A has a long period. It's going to resonate with that. Versus block C, Professor Lerr has this down to a science. He knows exactly how much to shake to make each one resonate. Very good. So period of the building versus period of the soil need to be dissimilar. We said in the lecture, a tall building has a long period. If you put it on bedrock, that's a short period. It's okay. If you take the tall building with a long period, you put it on clay with a long period, then you might get resonance and the damage is amplified. That's called amplification. Very good. So let's continue. This slide, we're done with this slide. So for taller buildings, there is something called a tuned mass damper. I should have put an animation in here. But basically, at Taipei 101, it's 760 tons. It's a huge weight, and it was lifted in each ring separately and then welded up between floor 95 and 100. And what happens here is it's on hydraulics and it's hanging. So it has 360 degrees of freedom depending on which way the wind is blowing. This tuned, tuned meaning the period of the building compared to the anticipated winds or earthquake, it's tuned to be different. So it dampens the movement of the building significantly. Tuned mass damper. Okay, the first tuned mass damper was at CityCore in New York City. And that one has a very interesting story that you need to probably read about, not for the AIE or anything, but you had this very tall building. It had to be a certain configuration because there was a church underneath it. And it had some kind of structure because the columns had to be in the middle of the facade instead of at the corners. So anyway, there was a tuned mass damper over here. And so if the building wants to go this way because of wind, the mass damper wants to go in the opposite direction.