L5 Foam Demo

Added: 2026-05-30

[00:00:00]hello everyone welcome to my lovely lovely apartment don't worry that column in the basement was fixed so I'm pretty sure the floor is not going to collapse so last week we talked a bit about axial forces and we're going to start with a little review so as you can recall axial forces are when we apply a force along the axis of a member in other words I'm going to pull up on this number and I'm going to apply a reaction down here in line with the force that I'm applying and if I pull I create tension if I push I create compression and just so you can take a look at this a little more closely when I pull the lines spread apart when I push they start to come together but then of course we have another mode of failure with compression and that is buckling and you can see that the buckling is occurring along the weaker Dimension the shorter dimension of this foam column of course I can change the capacity of this column by either supplying a different end condition so in this case we would be pinning both top and bottom and we would have a line of valve K would be equal to one but I can also fix the top of this column against Rotation by clamping down on it with my hand now you can see that the buckling shape is a bit different and I'm able to apply a little more Force before it starts to buckle I can also change the buckling length of this column and increase the capacity by applying some bracing and all I'm going to do here is just very lightly place my fingers on either side and you can see that once again the buckling shape has changed and in this case we now have essentially cut the length of the column the buckling length in half and by doing that we have increased the capacity of this column by four times and that's of course because an Euler's buckling formula the KL is squared so you can see that this foam is very flexible in other words I can apply very little load or stress and immediately get a noticeable amount of change in its life or strain so in other words this foam has a very low modulus of elasticity and that's going to be very useful for this demonstration because often terms like stress strain modulus of elasticity can be a little difficult to understand but if we can take a look at the deformation of this member under Loden hopefully those terms will start to click so now that we've reviewed axial forces this week we're going to take a look at perpendicular forces and so I'm going to take this member turn it on its side and apply two reactions at either end to support reactions then I'm going to take a weight or a force and apply it somewhere perpendicular to the length of this member a certain distance away from my support reactions and as we learned last week force times a distance is going to create twisting or moment about the axis coming this way of the member so if you take a look at the ends of this number you can see this rotation occurring and that rotation is happening about an axis that's coming out in this direction or in other words if I were to take this number and grip it just in my hands and try to apply that same type of shape to the member I would have to twist my hands around my thumbs like this you can see I create that same shape with the member so when we apply a force to a member a distance away from the support we create bending and of course the location of this weight changes the amount of bending that's applied so if I place this at Mid span that's where we get our most amount our maximum amount of bending however if I move it closer to the support you can see we get much less spending and again that's because bending is equal to force times distance so decrease the distance decrease the bending moment but it's important to note that we are not only creating funding in this member we are also creating shear and Shear is best understood by looking at a pair of scissors scissors work by taking two knives and placing them slightly offset from each other but very very close together and you can imagine that if I were to place my piece of foam between these two scissors and clamp down this piece of foam would fail very easily in Shear it would cut in half foreign if I rotate this piece of foam to show you these lines again I'm going to come a little bit closer and then if I apply that same rotation at either end to create bending in the member you can see that the lines that I've drawn at the top they become closer together at the bottom they spread further apart and so we're creating compression at the top and tension at the bottom where if I change my rotation now I'm creating tension at the top compression at the bottom there's also this line that I've drawn in the middle here and this is called the elastic neutral axis and that is where our bending stress is going to be zero there's no change occurring in the member along that elastic neutral axis and so if I were to plot the amount of stress in the cross-section of this member as I'm applying bending it would look like this where we have maximum tension at one side maximum compression at the other side and zero stress at the neutral axis we're looking at the piece of foam if I were to cut this somewhere as I'm bending it and take a look inside the member we would see maximum tension up here maximum compression down here and no stress in the middle I also know what happens when I apply too much bending to this member you can see at a certain point it pops and twists so if I turn it on the side here or you can see that pops and this popping we call lateral torsional buckling lateral of course means that it's happening out of plane of the member fortunal is what we call twisting and buckling as we learned before is how members fail in compression and as we just learned when we apply bending we're also creating compression and therefore we have the potential for buckling