# Determining a zero force member visually

• member 392791
In summary, the key to identifying zero force members in a truss structure is to look for unloaded joints where only two members meet or loaded joints where the line of action of the load coincides with one of the members. If there are three members meeting in an unloaded joint, then two of them are in a direct line with each other and the third member is a zero force member. In the case of four members meeting at a joint, it is not possible to determine if any of them are zero force members without considering the other joints they are connected to. However, it is also possible for a member in a complex structure to have zero force by chance.

#### member 392791

Hello,

When given a truss structure, how does one tell visually just by looking at the structure which members are zero force members?

From Wiki:

• "If only two members meet in an unloaded joint, both are zero-force members.

• If three members meet in an unloaded joint of which two are in a direct line with one another, then the third member is a zero-force member.

• If two members meet in a loaded joint and the line of action of the load coincides with one of the members, the other member is a zero-force member."

An unloaded joint is a joint where no external forces are applied , or a joint where there is no external reaction force.

A loaded joint is a joint where external forces are applied , or a joint where there is an external reaction force.

So that means if you have 4 members at a joint, then none of the members can be zero force members? I am looking at a homework problem and this is the case where 4 members meet and yet one or more of the members is a zero force member. I thought it was only 3 members meeting

Woopydalan said:
So that means if you have 4 members at a joint, then none of the members can be zero force members?

No, it just means you can't decide if they are zero force members by looking at that joint. But the other end of each member is connected to a different joint, and that joint might tell you it is a zero force member.

Of course it is also possible for a member in a complicated structure to have zero force just by happenstance, but that's not very interesting.

## 1. What is a zero force member?

A zero force member is a structural element in a truss or frame that does not experience any internal forces when the structure is loaded. This means that the member does not contribute to the overall stability or strength of the structure.

## 2. How can I visually determine a zero force member?

To determine a zero force member visually, you can look for three conditions: (1) the member is connected to three or more other members, (2) the member is part of a triangular shape, and (3) the member is not part of a loaded joint. If all three conditions are met, the member is likely a zero force member.

## 3. Why is it important to identify zero force members?

Identifying zero force members is important because it allows for a more accurate analysis of the structural forces. These members can be removed from the analysis, simplifying the calculations and providing a clearer understanding of the forces at work in the structure.

## 4. Can a member be a zero force member in one configuration and not in another?

Yes, a member can be a zero force member in one configuration of a structure and not in another. This is because the presence of external loads and the arrangement of the other members can change the forces within the structure, potentially causing a previously identified zero force member to experience internal forces.

## 5. Are there any exceptions to the rules for identifying zero force members visually?

While the three conditions mentioned above are commonly used to identify zero force members, there may be exceptions depending on the specific structure and loading conditions. It is always important to carefully analyze the forces within a structure and consult with an engineer if there are any uncertainties.