1. Project 2 – Part 3, Member Sizing please use these forces in your calculations!
B1 Mmax = k-ft
G1 Mmax = k-ft
C1 Pmax = k
127.5k-ft
127.5k-ft
-17k
334.8k-ft k k
k-ft
k-ft
-10.65k k
334.8k-ft
k-ft
k-ft

2. TRUSSES

3. What is a truss?

4. A truss is:
A lightweight frame used for relatively long spans
composed primarily of triangles, an inherently stable and non-deformable geometry
a flexible structure which withstands dynamic loads well

5. Long span, light, flexible, strong!

6. When would one use a truss?
Aesthetics? Structural capability? Architectural transparency?

7. 1. All members are straight.
Rules for trusses:
1. All members are straight.

8. 2. Members are pin-jointed at all connections.
Rules for trusses:
2. Members are pin-jointed at all connections.

9. 3. External loads and reactions occur at joints only.
Rules for trusses:
3. External loads and reactions occur at joints only.

10. 4. Axes of all members align to a single point at joints.
Rules for trusses:
4. Axes of all members align to a single point at joints.

11. Truss members following these rules can resist only axial forces:
TENSION
Trusses have a very efficient load path, as no members are in bending.
COMPRESSION

12. Typical truss formats found in the US:

13. In Project 1 you qualitatively analyzed 3-dimensional trusses.
Doing a quantitative analysis in 3-D can be quite complicated, so we will stick to 2-D “flat” trusses.
After all, many truss systems are largely 2-dimentional, or consist of many 2-D plane trusses parallel to each other.

14. “Analyze” a truss = to determine whether each member is in tension or compression, and with what magnitude, for a given loading condition.
The big picture: a truss is a composite spanning member, like a solid beam but made of many smaller parts to maximize material efficiency.
Solid beam:
Truss: P P C C C C C C C T T T T T T

15. V V P Solid beam: Truss: C C C C C C C T T T T T T C C C C C C C C T T
Ry1
Ry2 V C T C T V

16. Method of Joints: Draw free-body diagram of overall truss.
Determine reactions at supports, using SFx = 0, SFy = 0, SMany = 0.
Draw the free body diagram for each joint, one at a time.
Use SFx = 0, SFy = 0 at each joint to solve for member forces.
Go to an adjacent or neighboring joint and repeat.
Summarize all member forces in a diagram of the whole truss.

17. Method of Sections: Draw free-body diagram of overall truss.
Determine reactions at supports, using SFx = 0, SFy = 0, SMany = 0.
Make a cut through the truss that goes through the member(s) in question and draw this new partial free body diagram.
Use SFx = 0, SFy = 0, SMany = 0 for this partial truss to solve for member forces.

18. Project 2 – Part 3, Member Sizing please use these forces in your calculations!
B1 Mmax = k-ft
G1 Mmax = k-ft
C1 Pmax = k
127.5k-ft
127.5k-ft
-17k
334.8k-ft k k
k-ft
k-ft
-10.65k k
334.8k-ft
k-ft
k-ft