Steel Truss Calculation - The easy formulas you need to use

hi i'm robin deyoung and i'm a structural engineer i'm going to show you the very simple formulas you can use to design a long truss so let's just imagine here we have a very long span of 30 meters and it's perhaps a roof that we're going to support on this and the way that you span such a long length rather than say with a steel beam is by designing a truss so you might have something along these lines here so we have the supports at the end we have a steel truss and it's spanning 30 meters so the first formula i want you to think about and remember this is particularly useful in an exam situation is that the depth is equal to the span over 15 now that's just an approximate figure so 30 divided by 15 is 2. so if you're creating a preliminary size of a truss what you want to look at is the length or the span over 15 or over 18 some something like that so that's the first formula i want you to remember the second thing the second formula is if you have a load on here so there's a distributed load on the top call it w and it's equal to 10 kilonewton per meter now i use kilonewtons per meter that's sr units if you're designing in in europe you can obviously take if you're in america you can change that to the units that you need so we've got 10 kilonewtons per meter as a udl so to find out the moment so remember this formula as well and it's the same as a simply supported beam m is equal to wl squared over eight so in our case we've got 10 times thirty squared divided by eight so let's get the calculator for that one ten times thirty squared divided by eight one thousand one hundred five kilonewton meters so that's the moment so remember this formula here the next formula that you need is to find out the axial load on the bottom cord and the top chord so to find the force we take our moment and divide it by the length between the two chords so f equals m over l which is equal to one one two five kilonewton meters divided by two so we have 562 and a half kilonewtons of axial load in the top and bottom cord so [Music] that i'll just clarify that the figure we're talking about is is this here so in fact we'll call that d m over d where the depth is that there the depth between the chords which is two meters and the way to remember that is is that if you've got a moment with the units of 10 with the units killing you to meters if you divide it by a value of meters you end up crossing those two out you end up with a value for kilonewton which is what we've got so that is the third formula i want you to remember the next thing we need to find out is our value of i required the value of i is a value of stiffness so if we use this to limit the deflection and the deflection we're going to limit that to span over 360. so i've got a little formula here that i want you to remember i required is equal to 2.24 w l squared now that's a formula that i've derived by inputting l over 360 and values such as young's modulus to to derive this so it takes care of all the units so in our particular case 2.24 times w which is 10 kilonewtons per meter in this case that's total load so we're going to take 10 kilonewtons per meter times 30 is 300 kilonewtons times 30 meters squared 2.24 times 300 kilonewtons that's the total load times 30 squared equals 605 000 and the this formula gives you the output in centimeters to the four so that's the fourth formula that you will need to remember so there's a really neat little formula that will tell you the i value of a truss and it's approximate i is the area of the chord top or bottom chord multiplied by the depth squared over two so once you've sized your top and your bottom cord and you have a preliminary size for that based on the the force that it's able to the axial load that it's able to take you can then find out your eye value and compare it with your i required and just check that the deflection is not greater than span over 360.

Channel: Robin de Jongh

Views: 89,710

Keywords: Steel trusses design, design a roof truss, how to, how to calculate a steel truss, roof truss, roof truss calculation, roof truss load calculations, steel beam calculation, steel roof truss, steel truss, steel truss calculation, steel truss design, structural engineering, structural steel, structures, truss

Id: gNnyi8gxDQM

Length: 7min 28sec (448 seconds)

Published: Wed Sep 09 2020