L8 Moment Frame v Braced Frame

追加: 2026-05-31

[00:00:00]hello everyone and welcome back in this video we will discuss two types of lateral Force resisting systems the moment frame and the brace frame and to help us understand how these systems work without relying totally on mathematics we have our handy mola model using this model we can demonstrate how each of these systems actually provides resistance against lateral loads we can also make a rudimentary assessment of how well each of these systems performs under loading by giving them a shake test so let's start with what I have in front of me right now this is an example of a simple gravity frame consisting of two columns and a beam connected entirely by pinned connections in other words no bending can pass through these joints as you can see this model performs perfectly well under Gravity loading that is it can sustain its own self-weight without collapsing however when I apply lateral load by shaking the foundation you can see the structure starts to deform and if I were to remove my hand here it would collapse so how can we provide stability to this structure under lateral loading well notice in this deformed State how the angles between the members have changed these two angles have increased dramatically while these two have shrunk by the same amount so what if we were to stop that kind of distortion from happening let's give that a shot here I have some triangular magnets that I'm going to clip onto these joints and these clips are going to make these two joints now fixed instead of pinned in other words bending forces can now transfer through these joints and now as I apply load you can see that the angle between the beam and the columns at the Joint remains at 90 degrees even as I push and pull on this Frame note also what is happening to the rest of the structure here both the columns and the beams are distorting we can see this most clearly in the columns where the rings on one side are spreading apart and on the other side they are squishing together in other words we have tension on one side compression on the other this type of distortion is of course known as bending and I can give you an even better look at that Distortion by using this piece of foam here so if I take this piece of foam and I apply the same conditions to what was happening to those columns in other words there's going to be a pinned condition down at the bottom so I will allow this hand to rotate however I'm not going to allow it to translate back and forth and then vice versa with this hand it will remain locked against rotation however it will translate laterally and as you can see what's happening here similar to what we saw with the columns but now on a much larger scale you can see we have tension on one side compression on the other in other words bending so fixing these two joints was a success we created a frame with lateral stability when I push on it and let go it more or less returns to the same original position this Frame locks the joints between the beam and the columns enabling bending moments to form within the members this Reliance on bending in the columns and in the beam is where a moment frame gets its name now let's give this thing a shake as you can see it wobbles around quite a lot and this really shouldn't be much of a surprise to us we know intuitively that bending forces are very difficult for a structural member to resist especially long and slender members like columns and beams so how could we make this moment frame stiffer in other words how could we reduce how much it shakes well we know that bending resistance increases with depth right bending this foam in this direction is a lot easier to do than bending it in this direction again because the depth is greater when we Orient it this way compared to this direction so one option could be to increase the depth of these members in their plane of bending like this so these columns would get wider and this beam would get deeper and that would certainly be a valid solution however unfortunately I don't have the means to do that with this model for one and also Architects usually don't like the idea of wider and deeper structural members so let's try a different approach of course what I'm showing you here is just one frame and in the actual context of a building we would likely have many more frames so we would likely already have other Bays with gravity framing like this and if we wanted to increase the stiffness of this whole line we could just continue adding these fixed connections right so instead of having just one moment frame now we have two so now we have a second moment frame added here let's give this thing a shake again as you can see it still wobbles around quite a bit so I'm not really happy with that amount of wobble so can we make it stiffer still well unfortunately I've run out of these little Foundation elements and I've run out of room on this board so we're not going to be able to add another row of moment frame so instead I'm going to have to try something that would only ever really be considered as an absolute Last Resort in the real world of Structural Engineering as you can see we have already fixed all these joints up at the top here and you might have been wondering at the start of this video well why haven't we fixed any of the joints at the bottom wouldn't that provide lateral resistance well yes technically you're right adding these pieces here would provide greater lateral resistance however it's important to note that just because you can do something very easily very simply in a model doesn't mean that it will be quite as easy or simple to do it in the real world so in this model fixing these joints down at the bottom was very simple all I had to do was snap those pieces into place and it's done I've created more fixed joints however in the real world what we have below these joints isn't a nice solid piece of magnetic metal instead we have this soft compressible unpredictable mess of soil and creating a fixed condition in soil is extremely difficult and costly so we only ever do it when we absolutely must as is the case here since I ran out of parts so without further Ado let's give this a shake and as you can see it's not really moving around much at all success if I apply a static lateral load you can see a little bit of distortion but not nearly as much as what we were seeing before so I call this a success all right now let's move on to the brace frame all right so we're back to our simple gravity frame here now we know that a brace frame behaves very differently from a moment frame rather than relying on the bending resistance of the beam and columns we instead pass all of the lateral Force directly into this brace now all of the joints in a brace frame are going to be pinned as opposed to fixing the joints with the moment frame so in the case of the braced frame with all pinned conditions there will not be any bending whatsoever in any of these elements now it took a lot of extra material before we could get our moment frame system to stop shaking so much let's see how many of these braces it takes to achieve that same level of stiffness I'll start with the first one here and let's try applying Force already you can see it's not going anywhere even if I push on it in this direction this thing is not moving even if I shake it there is no movement whatsoever of course we shouldn't be that surprised by this if you imagine holding a long stick in your hand and someone asks you to break it would you do so by pulling on it or by bending it of course you would bend it and it's the same thing with these two lateral systems a moment frame is going to be significantly weaker by the nature of how it provides resistance through the loads it provides resistance through bending in the members whereas a brace frame acts completely axially so it's similar to trying to break something by bending it versus trying to pull on so hopefully that explains visually just how much more efficient a braced frame is compared to a moment frame system remember we had to add all of these joints plus an extra row of columns and we had to complicate the foundation just to achieve what the brace frame could do with this one skinny little strand so when it comes to cost comparison generally braced frames are far more cost efficient compared to moment frames