At times in truss structures and under particular loading configurations, some members do not carry the load. Such members are referred to as zero force member in truss. Generally, they are used to boost the strength of trusses and provide future supports with modification of the administered loading configuration.
So, what actually is a zero force members in trusses and why would you provide one? We’ve elaborated all the information below about zero force member:
Definition
A zero force member in a truss upon applying load remains at rest i.e. it neither takes tension or compressive load. In a typical configuration, you can find a zero force member at pin or connections where no external load exists and there are three or fewer members meet.
What is a zero force member in trusses?
Trusses are rigid engineering structures composed of long, slim members linked at their ends. They are usually utilized in spanning extended distances with lightweight but strong construction. Some common zero-force members truss applications include roof structures and bridges.
Occasionally, trusses have a single or more zero-force members. From the name, zero-force members do not carry any force and therefore do not support any load. They can be found on the application of equilibrium equations to joints, though spotting and eliminating them before beginning can save you some work.
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How To Identify The Zero Force Members In The Truss
When identifying zero force members, three inspection methods can be applied. Using these methods, all the joints in truss structures are analyzed one after the other.
Below are some essential tips in the determination of zero-force members in trusses
- For joints with only two members and no external support or load, then the two members can be said to be zero-force members
- If a loaded joint has two members only, then given the consequential force line of action from loads applied at the joints is on a straight line with any of the members, the rest of the members are zero-force members.
- If the joint has members without external support and/or load, then if two of the members lie in a straight line, the non-collinear member is a zero-force member.
To summarize, you can detect a zero force member at joints with two loaded or unloaded members and at joints having three members, whereby there is no exit of any load applied. The mission is focusing on the particular joint under examination.
What Are The Zero Force Member Rules
There are several rules through which we can establish whether members in a truss are zero-force members.
- Check for joints with three members. For example, if two of the members lie in the same line and the third is perpendicular to members one and two, and no external load is acting at the joint, the third member is a zero-force member.
- Similarly, with the third member at an angle, theta θ, from the first two members lying in the same line without any external force on the joint, the third truss element is a zero-force member.
- For joints with only two non-collinear members without external loads acting, then the two members are zero-force members.
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Function and Reasons to have one
- If you have a long slender member in a truss, it would probably fail in compression. To provide stability to the structure, you can use a zero-force member at the middle to avoid buckling.
- In trusses, the loading condition can change during the serviceability of the structure. For such situations, zero force members come handy while increasing the stability and rigidity of the truss.
Zero Force Members Examples
- Trusses are included in various structures around us. Examples are;
- Bridges – suspension bridges, cantilevered bridges, arch bridges
- Roofing – essentially every modern-day domestic housing. Both steel and timber use trusses in spanning ceilings
- Hangars – all big aircraft hangars or warehouses rely on the length and rigidity of truss roof design to span the large ceiling spaces without support.
- The zero-force members here do not offer load support, increase the stability at the time of construction, and add support if loading conditions are changed.
Just take an example of the truss structure below:
If a joint has only two non-collinear members and there is no external load or support reaction at that joint, then those two members are zeroforce members. In this example members DE, DC, AF, and AB are zero force members.
We can prove this assumption by applying equation of equilibrium to the joints circled as red:
For Joint A: sumof horizontal force = 0
so, Force AB = 0
and sum of vertical force = 0
so, Force AF = 0
Consider another example here:
If three members form a truss joint for which two of the members are collinear and there is no external load or reaction at that joint, then the third non-collinear member is a zero force member, e.g., DA.
During analysis of trusses, we can remove the zero-force members to make our calculation simple and easy.
Closing Quiz
So, I believe you now know enough about trusses, how about a quiz?
When supporting the same magnitude of force, truss members in compression are generally made _______ as compared to members in tension.
- Thicker
- Thinner
- The same size
[su_accordion][su_spoiler title=”Show the Correct Answer” open=”No” style=”default” icon=”plus” anchor=”” anchor_in_url=”no” class=””]The Correct Answer is 1) Thicker [/su_spoiler] [/su_accordion]
You can comment the write answer in the comments section below.
Conclusion
Since trusses are often designed to provide support to various loading conditions, finding members with zero forces is expected as we analyze trusses for a certain loading condition. The addition of zero-force members to trusses also braces compression members from buckling. Therefore, the identification of zero-force members helps expedite truss analysis.
For example, if joints have only two members that are non-collinear and without external support or load at the joint, then the two members can be said to be zero-force members.
