General: Lateral Systems

Okay, let me see if I can talk fast enough to let this video be less than 10 minutes. This one is about the lateral load resisting systems, and please understand that there's other videos that get into much more detail. This is just a summary, like a review for in-depth analysis of lateral load systems. So in summary, post and beam is gravitational only. And to resist lateral loads, including wind and seismic, in plan, we have something called a diaphragm. In elevation, we have moment frames, brace frames, and shear walls. So looking at the lateral resisting system in plan, which is the diaphragms, it comes in two varieties, a flexible diaphragm and a rigid diaphragm. A flexible diaphragm is typically made of pieces, be it OSB, plywood, precast concrete, metal deck without topping, all of these make up a flexible diaphragm. In addition, the aspect ratio or one dimension compared to the other dimension of the diaphragm is greater than 3 to 1. And the diaphragm is weaker than the vertical collectors. The vertical collectors are more rigid and they are stronger than the diaphragm. As a result, the diaphragm is the one that absorbs the lateral load and bends and deflects accordingly. There is no torsion in a flexible diaphragm because the vertical collectors are stronger than the horizontal diaphragm. Also, the load distribution, this one you must watch the video, but the load distribution from the horizontal flexible diaphragm to the vertical collectors that are stronger than the diaphragm is based on the tributary spacing of the collectors. So here are some flexible diaphragms. They include OSB, plywood, un-topped metal deck that has insulation and TPO or some other membrane but not cast-in-place concrete. hollow core planks or double Ts or precast members that do not have a topping are considered flexible. There are a bunch of pieces that in an earthquake start shaking around. Now, if there is a topping on top of it, it could be considered between rigid and flexible diaphragm. But still, in these double Ts, they're pretty strong. And then you put a two inch, three inch topping on top of it, these guys are much stronger than the topping and they might end up breaking it. So it's better to analyze it as a flexible diaphragm. Versus a rigid diaphragm is anything with cast in place concrete. And the diaphragm this time is stronger or more rigid than the vertical collectors. And as a result, it's the collectors that will be bending and deflecting, not the diaphragm, since the diaphragm is a stronger member. So with rigid diaphragms, the aspect ratio must be less than 3 to 1. If the aspect ratio is more than 3 to 1 and it has cast-in-place concrete, that does not qualify as rigid. The aspect ratio is too big to be considered rigid, even though it has cast-in-place concrete. So it's the vertical collectors that are going to bend and deflect since the diaphragm is stronger than the verticals. And torsion is definitely a consideration. The code specifies that as much as possible in lateral resisting systems, you must have the collectors placed symmetrically. If they are placed symmetrically and they have equal strength, then there still needs to be an account. The code requires an account for accidental torsion. So rigid diaphragms require a torsional analysis, even if the collectors are placed symmetrically. So torsion occurs when the center of mass and the center of rigidity are not concentric. There will be torsion. Load distribution, unlike flexible diaphragms that distribute the load based on tributary spacing of collectors, in rigid diaphragms, the distribution of lateral load to the verticals is based on the rigidity of the collectors, not just their spacing, but their rigidity. Very good. So anything with cast-in-place concrete, be it post-tension, be it regular, not post-tension concrete, be it topping on top of a metal deck, that makes it rigid enough compared to that deck. That one is flexible. So will this one on the top, there's going to be open-web joists and insulation metal deck. Okay, there's no cast-in-place concrete. That will be a flexible diaphragm on the roof, but the floor itself is a rigid diaphragm since it has cast-in-place concrete. Now, looking at in elevation, the lateral systems must be placed in a symmetrical fashion, both in rigidity and in placement, to avoid, to minimize torsion. Can't avoid it, but to minimize it. So in elevation, we have moment frames, we have brace frames, we have shear walls. Now, the brace frames come in two varieties, a concentrically braced frame versus an eccentrically brace frame. So let's go through these in order of ductility, because that's very important in an earthquake. So moment frames, here are some moment frames. are basically connected rigidly between the horizontal and the verticals. So they resist lateral loads by bending, be it cast-in-place concrete or be it a steel frame. A moment frame is the most ductile of the lateral resisting systems. And as a result, it's going to move the most. Therefore, it has the greatest drift. Materials, like I said, are cast-in-place concrete or steel with moment connections. If we take a closer look at this taller building, it has moment frames, and we can tell very quickly because of these web stiffeners. These are all moment connections. Versus if you look at this bay here, the column is continuous and the horizontal stops at the column. This is a gravity bay. But this one is a moment frame or laterally enabled lateral force resistance bay. So we have two bays of moment frame or lateral resistance ability. And then we have post and beam. You can tell because the column is much thinner than the column in the moment frame. The beam also is thinner than the beam in the moment frame. This tells us something that the moment frame is much more expensive than a brace frame or a sheer wall. But it comes with advantages. You don't have bracing in the facade. Therefore, real estate wise, it's a much more desirable system. But it cannot go very tall. because it drifts too much. So also with moment frames, they move a lot. They're very ductile. They drift a lot. Therefore, secondary damage. Ceiling tiles, pipes, air conditioning, all that is going to suffer because of drift and lateral movement. Excuse me. So these are some examples of moment frames. We have the Mies building in Houston, the Museum of Fine Arts, or Crown Hall at the campus of IIT in Chicago. These are moment frame buildings. And they are low rise versus high rise, only up to probably 15, 20 floors maximum with moment frame. Then you'll have to start bracing, adding bracing to the moment frame, or adding sheer walls. So that's the moment frame. Looking at concentrically braced frames, those guys, this one is a rigid, oops, this one has a rigid joint. That's how the load is transferred from horizontal to vertical by bending. The joint is rigid versus in a brace frame, the joint is pinned. So it's a post and beam plus a diagonal. And the load is transferred by tension or compression in that diagonal. And I've said many times before, tension and compression are axial forces and are much more efficient than bending. Bending requires a lot more material. That's why it's more expensive in the case of steel. It's a lot more expensive than bracing. Here are some examples of bracing. We have a chevron brace down here. We have an inverted chevron brace. This is a real funky building, but it's here just to show me something called an eccentrically braced frame versus a concentrically braced frame. The New York Times in Chicago, sorry, the New York Times in New York, of course, by Renzo Piano has these diagonal braces that are very nicely oriented. They don't intersect. But anyway, so these are concentrically, this is a concentrically braced frame versus an eccentrically braced. Had these come to the middle and intersected, we would have considered it more of a concentrically braced frame. But a concentrically braced frame is either an X or a single diagonal, but going from corner to corner. That's what concentrically means. Eccentrically is when the diagonal doesn't go to the corner and it leaves something called, let me erase here, it leaves something called a link beam. So this beam here, from here to here, is not braced. This one is not braced and it's there to bend. And the longer that link beam, the closer you are to moment frame. The shorter that link beam is, if they were intersecting, then there is no link beam, then it's closer to concentrically braced frame than it is to moment frame. So, eccentrically braced frame is exactly between a moment frame and a concentrically braced frame. So, in summary, concentrically braced frames have pin joints, not rigid joints, and they resist lateral load by axial tension or compression, not by bending like a moment frame. They are less ductile as a result. They don't move as much. They are more rigid than a moment frame. Therefore, the drift is less. And these are made typically, concentrically braced frames are made of wood or steel, not concrete. Because concrete could go into tension, we have a problem. So moment frames are done in concrete and steel, but cannot be done in wood because you cannot make a moment connection. And brace frames are done in wood or steel, but not concrete. And a concentrically brace frame, since it does not drift as much, is going to have less primary and secondary damage compared to a moment frame. And a brace frame is less expensive than a moment frame. With the absence of diagonals, the beams and columns we saw in this image are much fatter than if they were braced. If they were braced, then the beams and columns are going to be thinner, which means they're lighter. Steel is sold by weight, therefore it's cheaper. Very good. So the last one to talk about is a shear wall. And a shear wall could be masonry or concrete or it could be plywood or it could be OSB, but not steel. So if you have a steel frame and you go in there, if you have a steel frame with a post and beam condition, and you go in there and you put some kind of rigid infill, be it masonry, be it concrete, Then this one has pin joints, but you're doing a rigid infill. And therefore, you just made it rigid in this direction. Or you can attach it to a concrete wall. Let me erase. Or you can attach the post and beam to a concrete wall or a masonry wall. And now we have provided enough lateral support for that post and beam not to rack. Please remember there's three ways of securing a frame to lateral forces. One is a moment frame, two is a brace frame, and three is the one we're looking at, a shear wall. So in a shear wall, it's typically made of concrete. Yes, it could be plywood or OSB. It resists lateral loads in shear. So at Wrightwood 659 in Chicago, we can see that Tadao Ando inserted these beautiful shear walls incisioned inside of a masonry structure, historic masonry structure. Very beautiful, but of course it can handle loads in its plain shear wall, so it stabilizes the floors. Likewise, at the Seashore Library in China by Vector Architects, a beautiful concrete building, and any of these cast-in-place concrete walls serve as a shear wall. So, so far, we have bending. Moment frames are resisting lateral load with bending. Braced frames are resisting lateral load with tension or compression axially. And shear walls are resisting lateral load with shear, through shear forces. Okay, so this one, the shear wall is the least ductile, and it's the most rigid. And it experiences the least amount of drift, and it could be made of wood or concrete, not typically made of steel. So it's important to understand the qualities of each of these lateral resisting systems, again there's videos, extensive videos that I've created to get into more detail on this topic, but these are just highlights of the systems.