Structures

Earlier, external forces applied to a body were of the primary interest. However, real world structures typically have multiple members interacting via internal reactions. Internal forces in different members are of great interest.

Definition: Engineering structures are connected systems of members supporting or transferring forces.

Common types of structures: Trusses, Frames, Machines.

Generally, analysis will be done by dismembering the structure and analysis of equilibrium of individual members.

Plane Trusses
Truss is a framework composed of members joined at their ends to form a rigid structure (see examples in Fig. 6.5 in the book).

Applications – bridges, roof supports, derricks, etc.

Members: I-beams, angles, bars, special shapes fastened by welded, riveted and bolted joints.

Plane Truss – all members lay in one plane. Often they are used in bundles (pairs).

Basic Element – Triangle, the only stable rigid polygon (6.3).

Plane Bridge Trusses, Examples

Plane Roof Trusses, Examples

 

Assumptions In Analysis

• All members are two-force members.

 

• All forces are applied at joints – pins.

• Weight is negligible, when compared to other forces. However, if weight has to be accounted it is done through w/2 applied at the ends of each member.

 

General Procedure

Usually equilibrium of the whole structure is considered with respect to external forces to determine reactions. However, external reactions are not always needed.

There are two methods for finding internal forces in trusses:

Method of Joints (MoJ)

Consider FBDs of joints: applied forces are concurrent. Therefore, there are only 2 equations of equilibrium per joint – reduced complexity due to an absence of moment equations.

• Start with any joint with at least one known force and no more than 2 of unknown forces.

 

• Repeat this step iteratively through the rest of the joints until the required member(s) is reached.

Joints – Special Cases

Notes:

• Mark joints by letters and forces/members by two letters.

• Tensile force should point away from members. Compressive – inside.

 

• Cross-check your solution: when a joint connected to external supports is reached, one can compare MoJ solution to reaction computed from FBD for the whole structure.

Consider if you need external analysis at all.

Make a joints list with numbers of knowns and unknowns. Create a plan of analysis.

Use elimination techniques to avoid simultaneous equations.

Advantages / Disadvantages Of MoJ

Advantages of the method of joints: simple, no moments considered.
 

Disadvantages of the method of joints: slow and computationally expensive for large trusses (even if you need forces in just one or two members).

To overcome the difficulty, another method is used often: Method of Sections.

Method of Sections (MoS):

When solving a problem, the structure is cut through members instead of joints and each sub-part is treated as a RB.

Condition for static determinacy for 2D: cut should go through no more than 3 members, otherwise there will not be enough equations for finding unknowns.
 

Problem solving notes:

• Determine whether external analysis is necessary?
• Analyze the portion of the truss with minimum number of external forces.
• Assign force direction in cut members by inspection or arbitrarily. In the latter case make final assignments by sign of computed forces.
• Use elimination techniques to avoid simultaneous equations, particularly ElimiationTechnique.

 

Advantages / Disadvantages Of MoS

Advantage: fast, via making cut through the members of interest.

Disadvantage: in most cases have to analyze moment equilibrium.

MoJ and MoS can be combined!

When the member of interest is a central member of a large truss, and there is no cut passing through this member and crossing ≤3 members with unknown forces, apply MoS to find internal forces in members near the member of interest. Then, use MoJ to find internal forces in the members of interest (example problem 4).

Multistep (sequential) MoS: in the same situation, MoS can be applied several times (see for example sample problem 6.2 in the book).

Two-Force vs. Multiforce Members

Trusses, studied earlier, are made of two-force members only, when resultants of forces applied only at ends have to share a common line of action.

However, structures are not restricted to two-force members pinned together at the ends.

Multiforce Members
Unlike two-force members, multiforce members have more than two forces applied to them, or at least one couple and two forces.

Typical examples:

There can be forces applied not at the ends.

Rotation might not be allowed at one of the ends.

Implications:  we don’t know direction of forces right away!! MoJ and MoS cannot be applied

 

 

Frames
Definition: Frames are structures with at least one multi-force member.

Basic approach: To solve problems with frames one has to, generally, dismember the frame and analyze members or groups of members individually in turn.

In 2D there can be max 3 unknowns per member. In 3D there can be max 6 unknowns per member.

Problem solving notes

Determine whether you need reactions from external analysis?

Draw FBD’s of members in their respective positions. This helps with appreciation of active/reactive forces.

Use elimination techniques for individual members.

If external force is applied at the joint, show it only acting on one member connected to the joint (see sample problems 6.4-6.6 in the book).

Assign force directions arbitrarily or by inspection. Make final assignment based on computed signs.

Look for two-force members, it will greatly simplify solution via known direction of forces: in them and connected members.
Note: two force members can be curved (see notes on equilibrium of two force bodies)

Look for symmetry. Symmetrical cases often provide significant simplifications by allowing to judge magnitude/direction of forces.