NASA-Threads
Work and Mechanics
Lesson 51: Truss Design Challenge
Truss Design Challenge
A paper truss is to be designed to achieve the maximum load capacity to truss weight ratio.
Loading will be applied at the supports and at center of the top cord as shown above. Each
group will be supplied with:



mat board for the compressive members,
manila folder material for the tensile members, and
3/16” diameter aluminum screw posts to secure the joints.
Schedule (teams of 2 or 3):
Class #
1 (today)
2
3
4
5
6
In-Class Activity
Truss design challenge &
Example
Truss design
Truss design and fabrication
Truss fabrication
Truss testing
Truss presentations
Homework
Preliminary truss design
Design work & fabrication
Design work & fabrication
Truss fabrication
Presentation
NASA-Threads
Work and Mechanics
Lesson 51: Truss Design Challenge
Design Rules:
1. The structure to be tested must be a truss that can be analyzed using the method of
joints.
2. The truss must be loaded at a joint in the center.
3. Two joints separated by the required span must be present and able to be
supported from below.
4. The competition is designed so that truss failure will occur in a tensile member.
5. The truss must be built using the materials provided to teams by the instructor (you
can’t use your own manila folder or mat board).
6. The maximum combined length of all truss members is five times the span. This
includes tensile members and compressive members, where member lengths are
measured between the centers of joints.
7. Tensile members must be made of manila folder material, and compressive
members must be made of mat board.
8. The tensile truss members must have a maximum width of 0.25 inch in the center
and a maximum width of 0.50 inches elsewhere. The “narrowed region” tensile truss
members must be at least 1 inch long.
9. Up to two tensile truss members may be doubled to improve the strength of the
most critical members based on truss calculations. These two parallel members
cannot be glued together along their lengths.
10. Extra segments of mat board may be glued to each side of the compressive members
to keep them from “bowing” or “buckling.” These extra mat board segments must be
at least two inches shorter than the length of the member.
11. Teams can use permanent markers or pencils to decorate their trusses, but the truss
can’t be painted, dipped in glue, or treated in other ways to improve the strength of
the paper.
12. The truss must fit in the testing device and move freely between the clear acrylic
side plates. That is, the side plates may not be tightened down to pinch the truss
during testing.
Testing:
1. The instructor or others appointed by the instructor will serve as the judge in this
completion.
2. The trusses will be tested using the truss testing device provided by the instructor.
3. The trusses will be loaded using the weights provided by the instructor (such as 1 lb
bags of rocks).
4. The strength of a truss will be recorded as the last weight that was held for at least 2
seconds.
NASA-Threads
Work and Mechanics
Lesson 51: Truss Design Challenge
Scoring:
1. Tensile truss members must have a width of 0.25 inches or less. To discourage
“upward fudging” the width of tensile members to increase truss strength, the total
0.25
load that the truss carries will be reduced by multiplying it by 0.25+1.2(𝑒𝑥𝑡𝑟𝑎 𝑤𝑖𝑑𝑡ℎ). For
example, if the widest tensile member in the truss is 0.30 inches and the truss
0.25
breaks at 15 lbs, then the load will be multiplied by 0.25+1.2(0.3−0.25) = 0.806,
resulting in a truss strength of 15 lbs x 0.806 = 12.1 lbs. The judge will decide on
conformity.
2. Trusses will be weighed using a scale provided by the instructor.
3. The team with the maximum truss strength to weight ratio will be declared as the
winner:
𝑇ℎ𝑒 𝑜𝑏𝑗𝑒𝑐𝑡𝑖𝑣𝑒 𝑖𝑠 𝑡𝑜 𝑚𝑎𝑥𝑖𝑚𝑖𝑧𝑒 𝑡ℎ𝑒
𝑡𝑟𝑢𝑠𝑠 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ
𝑟𝑎𝑡𝑖𝑜
𝑡𝑟𝑢𝑠𝑠 𝑤𝑒𝑖𝑔ℎ𝑡
Truss Design Example
The truss tested in the video viewed earlier was analyzed using the method of joints where
a load of “1 lb” was assumed to be applied at the center. The numbers listed beside the
members are the computed member loads. The full analysis is provided at the end of this
set of notes for reference.
B
0.707 lb (C)
A
45
6 in
1 lb
1 lb (C)
C
0.707 lb (T)
D
0.707 lb (C)
0.707 lb (T)
0.5 lb (T)
1.5 lb (T)
0.5 lb (T)
G
6 in
1 lb (C)
F
span = 18 in
0.707 lb (C)
E
NASA-Threads
Work and Mechanics
Lesson 51: Truss Design Challenge
Member FG will experience 1.5 lbs of tensile force for each lb of force applied at joint C. If
the tensile capacity of member FG is 9 lbs (based on our tensile testing), then . . .
𝑝𝑟𝑒𝑑𝑖𝑐𝑡𝑒𝑑 𝑡𝑟𝑢𝑠𝑠 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ =
𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑚𝑎𝑛𝑖𝑙𝑎 𝑓𝑜𝑙𝑑𝑒𝑟 𝑚𝑒𝑚𝑏𝑒𝑟𝑠
9 𝑙𝑏𝑠
∙ 1𝑙𝑏 =
∙ 1 𝑙𝑏
𝑚𝑒𝑚𝑏𝑒𝑟 𝑓𝑜𝑟𝑐𝑒
1.5 𝑙𝑏𝑠
= 6 𝑙𝑏𝑠
If we double this member by using two parallel manila folder strips (which is allowed by
the rules), then the strength of member FG is really 18 lbs, and the truss is expected to
break at a load of 12 lbs. We recommend doubling the critical members!
The table below summarizes the above calculations for all members. The strength of the
compressive members is left blank since they would fail by buckling if the side plates of the
testing device did not restrict them from bowing. The compressive members are not
expected to control failure for our trusses.
member
tensile or
force
Member compressive?
(lbs)
AB, DE
BC, CD
AG, EF
FG
BG, DF
CG, CF
C
C
T
T
T
C
0.707
1
0.5
1.5
0.707
0.707
member
tensile
capacity
(lbs)
number of
members?
(1 or 2)
predicted strength of
truss assuming this
member controls failure
(lbs)
9
9
9
9
9
9
--1
2
1
--
--18
12
12.7
--
This table shows that member FG is critical and is expected to fail when a load of 12 lbs is
applied to joint C. This analysis corresponds to the same truss shown in the video viewed
earlier. That truss broke at 13.4 lbs which is very close to the predicted 12 lb breaking load.
The percent difference between the predicted and actual failure loads is computed below.
% 𝒅𝒊𝒇𝒇𝒆𝒓𝒆𝒏𝒄𝒆 =
𝟏𝟑. 𝟒 𝒍𝒃𝒔 − 𝟏𝟐 𝒍𝒃𝒔
= 𝟏𝟏. 𝟕%
𝟏𝟐 𝒍𝒃𝒔
This shows that statics, and the method of joints, can be used to accurately predict the
failure load of a truss!
NASA-Threads
Work and Mechanics
Lesson 51: Truss Design Challenge
Truss Calculations
The calculations below provide an example of how to design a truss.
6 in
C
B
A
1 lb
D
45
E
G
6 in
F
span = 18 in
NASA-Threads
Work and Mechanics
Lesson 51: Truss Design Challenge