Forces in Members of a Roof Truss

Dear viewers, Today I have come up with yet another helpful tutorial for civil engineers. Civil engineering is my field and I am in love with it. I prefer to share my little knowledge with others to help them in their studies and help them to fix problems faced by them. So coming towards today’s topic, engineers are well aware of what is a Truss. Truss is a structure that carry the loads applied to it and consists of chords which may be made up of steel or wood. Trusses are of different shapes and types, for different structures such as roof, bridges etc. Each type has different load distribution. The truss member are connected to each other with the help of joints. The truss I have chosen for illustration of the calculations of forces is called a Fink or French Truss. This truss has 27 members. We are going to find out that which member is to be designed as a compression member and which one as tension member. Simply, we will find the maximum factored Compression Force and maximum factored Tension Force in each member then we will decide further about the designing of these members.

Here is the truss with joints labelled as A,B,C and so on. Members have been labelled as M1, M2, M3 and so on.

truss member forces

 The excel sheet I have posted is explained below. It will guide you to make your own. First of all input all the data shown here.

truss member forces

Enter the values of forces under unit gravity load, unit wind load on hinge side and unit wind load on roller side in columns 2, 3 and 4 respectively. (Note: these values of forces can be find out by solving the truss by the method of joints or section or by a software called Risa 2-D, I will illustrate these calculations in my next Tutorial about this software). Then write down the formulas in the columns 5,6,7,8 and 9.

truss member forces

The maximum value out of these columns (5,6,7,8,9) will determine the Maximum Factored Tension and Compression in columns 10 and 11 respectively.

truss member forces

truss member forces

Below is the excel sheet for the calculation of forces in truss members. The  formulas used in it are applicable for any shape of truss. You can edit this sheet with your own values.

Excel Sheet for Table of Forces

Also see Excel Sheet for Beam Design on my blog. Stay tuned for more useful stuff. Subscribe to us for our updates. Thanks for visiting Knowledgeoverflow.


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