How to Determine Zero-Force Members in Truss

by ce on March 19, 2011

Sometimes in truss structure under certain loading configuration there are some members that do not carry the load.  These members are called zero-force members and normally used to increase the stability of truss and to provide future supports in the case of the applied loading configuration is changed. These zero-force members can be determined using method of inspection. In this method, we basically inspect or examine each joint in a truss structure one by one. Followings are some guidelines that can be used to help finding the zero-force members in truss structures:

  1. If a joint only has two members and no external load and/or no support, then those two members are zero-force members.
  2. If a joint only has two members and is loaded, then if the line of action of resultant force from applied loads at the joint colinear with one of the members then the other member is a zero-force member. If the resultant force at the joint is not colinear with either member, then both members are not zero-force members.
  3. If a joint has three members and no external load and/or support, then if two of members are colinear then the non-colinear member is a zero-force member.

As summary, zero-force members can be found at a joint with two members with or without applied load(s) and at a joint with three members in which there is no applied load. The key is to concentrate on the particular joint being inspected. Don’t worry about conditions at the other end members being inspected at particular joint.

{ 11 comments… read them below or add one }

origbemide July 13, 2012 at 7:44 pm

the note here was found very useful and explanatory.
thank u.

Josh December 4, 2012 at 8:26 pm

Very nice. You should link to your example page from this page.

Also, it’s spelled collinear. ;o)

Red February 4, 2013 at 3:30 pm

I can.t say nothing, It is morthan I expected THANKS!!

nirup January 7, 2014 at 6:52 pm

thank you very much!!

ce January 9, 2014 at 5:11 am

You are welcome!

Kelsie March 25, 2014 at 3:32 pm

What if a joint has 4 members and no load?

ce April 10, 2014 at 1:24 am

Then we cannot determine if there is any zero force member.

davs August 16, 2014 at 3:47 am

what if there are ten members and the resultant force is collinear with one member, should we cancel the other nine member?

davs August 16, 2014 at 3:49 am


ce August 17, 2014 at 1:09 pm

Nope, unless all the other nine members perpendicular with the resultant force (impossible) …

Karar January 16, 2016 at 10:53 pm

We don’t used to find the resultant of jointed forces, the second role is not obvious.

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