Steps to design a steel truss : 1) Calculate first the loadings to be carried by the truss. These includes dead loads (weight of roofing including purlins, lightings, ceilings), live loads (normally for residential is 40psf) and wind load. The resulting loadings will be in N/m^2. 2) Multiply the resulting loadings by a tributary area of a roofing, which is a spacing of the purlins. (Example : 10 N/m^2 x 0.60m, the 0.60m is the spacing of purlins). The result will be in N/m which is a uniformly distributed load to be carried by purlins. 3) Calculate the reaction at the support of purlins (support of purlins is truss). These reactions will be the concentrated loadings P to be carried by truss top chords. Take note the value of concentrated loads at both ends of a top chords is only 1/2 of the value of P. 4) Draw the free body diagram of the truss. Calculate the reactions at the support of truss (usually beam). Show all the value of loadings, reactions and direction of loadings in the free body diagram. Dont forget to have a proper scale of all the loadings and the truss diagram. 5) From the free body diagram, use the Maxwell Diagram to be able to determine the stresses in each members of truss. Stresses are either compression or tension. These stresses will be used to design the truss members. 6) To design the truss members, treat them and design them as axially loaded columns. There are 5 formulas to be used to design truss members depending on the result of (L/r) of each member. L = length of member, r = radius of gyration. r = (I/A)^2, where I = moment of inertia and A = assumed cross sectional of member 7) The 5 formulas are : A) Rankine Gordon Formula (when L/r is less than 160 but greater than 60) P/A = 124 / (1 + (L/ r)^2/10000); P is the allowable load to be carried by member and A is the cross sectional area of a member Straight Line Formula (when L/r is less than 120 but greater than 30) P/A = 110 - 0.483(L/r) C) AISC Formula (when L/r is less than C), where : C = ((2 x (Pi)^2 x E) / fy)^1/2 P/A = (1 - (L/r)^2 / 2xC^2) x (fy/ES), where : ES = 5/3 + 3(L/r)/8C - (L/ r)^3/8C^3 (when L/r is greater than C) P/A = (12 x Pi^2 x E)/(23 x (L/r)^2 D) Old AISC Formula : (when L/r is less than 120 P/A = 119 - 0.0034 (L/r)^2 (when L/r is greater than 120 but less than 200) P/A = (7.30 x 10^4)/ (1 + (L/ r)^2 / 7.30 x 10^4) E) Conventional Method Fa = P/A + Mc/I (where c is the height of the nuetral axis of steel section) NOTE : That after having the value of P/A, check and compare it against the allowable stresses (compression, bending or tension). If value of P/A is greater than the allowable stresses, then your assumed section for steel truss members are safe. If the value of P/A is less than the allowable stresses, then redesign and assume another section calculate again
Posted on: Wed, 23 Oct 2013 14:34:57 +0000
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