Trusses

Question 1

What is a truss or frame?

Answer 1

A set of pinned connected axial members.

Question 2

What are joints in a truss or frame?

Answer 2

The connection points of the axial members.

Question 3

What is disregarded in trusses and frames?

Answer 3

The weights of the members.

Question 4

Where are loads applied on trusses and frames?

Answer 4

Only on the joints.

Question 5

What does a structural cell consist of?

Answer 5

It consist of all the members in a closed loop of members.

Question 6

What is needed for the truss to be stable?

Answer 6

All of the structural cells must be triangles.

Question 7

Why are trusses analyzed as two dimensional structures?

Answer 7

Truss loads are considered to act only in the plane of a truss.

Question 8

What are internal forces in trusses?

Answer 8

Forces in truss members hold the various parts together.

Question 9

How is a free body diagram drawn for trusses?

Answer 9

A free body diagram of the joints or pins is drawn.

Question 10

In what direction do the forces point for a pin in compression?

Answer 10

Towards the pin.

Question 11

In what direction do the forces point for a pin in tension?

Answer 11

Away from the pin.

Question 12

Why are the forces on the truss joint concurrent forces?

Answer 12

Because the truss members are axial members.

Question 13

What kind of equilibrium needs to be forced at each pin?

Answer 13

Only force equilibrium

Question 14

What are zero force members?

Answer 14

A third member framing into a joint already connecting two collinear members carries no internal force unless there is a load applied at that joint or Both members forming an apex of the truss unless there is a load applied at the apex.

Question 15

When is a truss statically determinant?

Answer 15

When the number of members is equal to the product of 2 and the number of joints minus 3.

Question 16

When is the truss statically in determinant?

Answer 16

If the number of members is greater.

Question 17

When is the truss unstable?

Answer 17

If the side with the number of joints is greater.

Question 18

When will an unstable truss collapse?

Answer 18

Under certain types of loads.

Question 19

What is a plane truss?

Answer 19

A rigid framework where all the truss members are within the same plane and are connected at their ends by frictionless pins.

Question 20

In where and in what plane are external forces applied on trusses?

Answer 20

External loads are in the same plane as the truss and are applied at the joints only.

Question 21

What do the truss reactions and member forces have to be equal to, to be statically determinant?

Answer 21

Zero.

Question 22

What is the method of joints?

Answer 22

One of the methods that can be used to find the internal forces in each truss member.

Question 23

When is the method of joints useful?

Answer 23

When most or all of the truss member forces are to be calculated.

Question 24

When is the method of joints not useful?

Answer 24

When single isolated member force is to be calculated.

Question 25

How does the method of joints work?

Answer 25

It is a direct application of the equations of equilibrium in the x and y directions.

Question 26

What are the steps in the method of joints?

Answer 26

The method begins by finding the reactions supporting the truss. Next the joint at one of the reactions is evaluated, which determines all of the member forces framing into the joint. Then, knowing one or more of the member forces from the previous step, an adjacent joint is analyzed. The process is repeated until all the unknown member quantities are determined.

Question 27

How is the sense of force determined in a member?

Answer 27

If the sense of force cannot be determined by logic, an arbitrary decision can be made.

Question 28

Why would a sense of force be negative?

Answer 28

If the incorrect sense of a force is chosen.

Question 29

What is the method of sections?

Answer 29

A direct approach to finding the forces in any arbitrary truss member.

Question 30

When is the method of sections useful?

Answer 30

When only a few truss member forces are unknown.

Question 31

What are the steps to the method of sections?

Answer 31

The first step is to find the member reactions. Then a cut is made through the truss, passing through the unknown member. Finally, all three conditions of equilibrium are applied as needed to the remaining truss member portion.