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TRANSCRIPT
JACQ UI DA NIEL I RYAN TRETOW I A LI TRUWIT
PRELIMINARY BRIDGE DESIGN:
We designed our bridge to be efficient and structurally sound rather
than focusing on aesthetics. We began by designing the bridge within
the dimensional parameters of the assignment, and then further
reduced the depth and weight of the bridge to maximize the bridge
efficiency and give Team Bridgiant the greatest opportunity to win.
Our bridge is designed to hold 90+ lb load at a weight of less than 4
FURTHER DESIGN DEVELOPMENT:
With further development, we realized we would need to maximize
the stability of our bridge. To do so, we added gussets plates to all ·~---of our member joints to reduce stress at the joints and allow for
a stronger connection from member to member. Additionally, we
devised ~ ed, laminated member method to enhance the strength
of our lower arched member and create a continuous bent member:
ounces. Our bridge is composed of 37 total members; 20 of which effectively reducing the number of joints and individual member in
comprise the perimeter of the bridge and are 1/4"x1/4" . The remaining the total bridge assembly. We also aaded 1" tabs on the end of our
members are 1/8"x1/8", allowed the bridge to support a large amount truss assemblies to aid in the secure fitment of our bridge within the
of weight while remaining very lightweight. Further, we designed the official testing frame, which provided solid footing for our bridge to
bridge to be inverted to leave a clear, open roadway deck to stack the hold weight beyond what was initially anticipated.
winning amount of weight. The oridge rests on two fixed supports
that can resist vertical and horizontal forces as well as the moment(s) After initial design of our bridge, Me also concluded that we would
at each end. These fixed supports are representative of the provided need to find additional weight reduction strategies to ensure our
testing frame. bridge met spec. To reduce weight. we prepared an ultra-thin
MEMBER SIZING:
20 - 1/4" x 1/4" Basswood Members
17 - 1/8" x 1/8" Basswood Members
WEIGHT:
Approx. 3.8 oz
+ 10% For Glue Weight
EXPECTED MAX LOAD:
90+ lbs
·~
road decking solution and strategically placed cross members for ·-----=----maximum strength and resistance to torsion while controlling and
monitoring overall weight of bridge construction. As a result, our
team precisely met the weight specifications at 4.0 ounces.
BRIDGE ADJUSTMENTS:
Besides the edits noted above, our initial bridge design was largely
satisfactory based on the results of the preliminary report. Therefore,
our initial design remained relatively unchanged.
MEMBER SIZE ADJUSTMENTS:
Our member sizes, placement and quantity met bridge weight
spec and exceeded carrying ca acity requirements so we chose
to maintain our member sizing strategy and design in an effort to
maximize the bridge 's ability to hold a winn ing amount of weight.
IMPACT OF PRE-ANALYSIS ON FINAL DESIGN:
After initial design of our bridge, we also concluded that we would
need to find additional weight reduction strategies to ensure our
bridge met spec. To reduce weight, we prepared an ultra-thin
road decking solution and strategically placed cross members for
maximum strength and resistance to torsion while controlling and
monitoring overall weight of bridge construction. As a result, our
team precisely met the weight specifications at 4.0 ounces.
We were planning for the bridge to maximize its weight to strength
ratio to the greatest extent possible and therefore initially weighed
in at 4.2 ounces. Through a bit more weight reduction measures
at the location of gussets, we were able to meet the final weight
requirement and offset the final unknown of how the glue weight
would add to the total weight of our bridge in the end. In summary,
our careful preparation during the preliminary design phase, we
were able to execute very accurately on our initial proposal. The
carefulness of our initial efforts allowed us to efficiently construct a
successful bridge and have confidence that we as a team, and our
bridge, would perform excellent under pressure.
REVISED BRIDGE DESIGN ANALYSIS:
MEMBER SIZING:
20 - 1/4" x 1/4" Basswood Members
17 -1/8" x 1/8" Basswood Membey
WEIGHT:
Approx. 3.8 oz
+10%~eight -EXPECTED MAX LOAD:
90+ lbs
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DERIVATIONS OF MEMBER AREAS:
1/8"
Compression to Grain (T) 5.78
Compression to Grain (II) 73.91
Tension to Grain (II) 70.31
Shear to Grain (II) 15.47
1,'4"
23.1 25
295.63
281.25
61.86
PREDICTION OF CAPACITY:
Based on this revised design, the bridge could hold a total weight of
901bs
MODE OF FAILURE:
Based on the below calculated conditions, the bridge will fail at the
bottom member of each end reaction location
Estimated Weight 3.0 ounces + 10% Glue Weight
d...-eD~fH17fcJ ~ 11tdJ (
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11
MEMBER SIZING
FROM AVAILABLE STOCK:
JOINT CALCULATION: 6.ll lB / 3~ 3 "-2 + 2,1 A 2 = 3,65 A 2
SLOPE= 2.1 / 3
... USING SIMILAR TRIANGLES: Fx = 40.447 lb
... USING PYTHAGORE S THEOREM: Fy = 81.1 lb
-\0
CRUSHING CALCULATIONS:
With 1/8" members, our weakest member is
the lower member at the location of the end
reactions, which is the member calculated at
the left. If utilized as shown and calculated, the
bridge would fail at this location and have a ~
capacity of 90 lbs of total weight. This is based
on Dr. Frame illustration shown above as well as
derivation of member areas listed above on this
page.
F=P/A 7
ILLUSTRATION OF TESTED (REVISED FROM PRELIMINARY) DESIGN
*SEE DR.FRAME FOR MEMBER STRESSES
ELEVATION
PLAN
4.rr
SECTION
2.69"\1 ' X1/4")
TESTING RESULTS
WEIGHT GR
WEIGHT OZ
HEIGHT
LOAD CAPACITY KG
LOAD LB
MEMBERS
JOINTS
PANELS
HEIGHT /LENGTH_ RATIO
LOAD/WEIGHT_ RATIO
TYPE
SCORE
PICKS
OBSERVATIONS OF TESTING:
11 3.398
4
6.5
56.699
125
44
4
10
0.216667
1.95312
Deck Bridge
72.5
20
The bridge successfully held iµell beyond the minimum requirement
of 50 pounds, with a total capacity of ~ nds. We utilized a
strategy of evenly placed weight blocks during the testing period, and
in hindsight see the potential to place the weight differently to reduce
overall stress on bridge trusses. Perhaps, had we placed more weigh
towards the ends, instead of a large number of weights at the center,
we may have been able to hold a greater number of 51b bricks.
BEFORE
AFTER
DESCRIPTION OF MODE OF FAILURE:
The bridge tailed as expected, due to a strategic reduction of cross
members and non-critical members to ensure that the bridge
efficiently met the overall weight requirement. Due to this, we were
able to predict the areas of fai lure (via torsion of the bridge assembly)
and as such, the bridge failed at the I cation of the horizontal and
angled between-truss cross members.
Based on the integrity on our pair of truss assemblies (based on
initial analysis in Dr. Frame) we knew it would be critical to place the
weights as carefully as possible to red ice the above mention effects.
We were able to do so up to 125Ibs in total weight, before the weight
stacks got too tall to be resisted by truss assembly. We know this
based on resulting integrity and limited failure of the arched trusses
and associated member connections. and the specific and focused
failure of the bridge at the area of the ultra-thin deck and limited cross
sectional width bracing.
FOLLOWING THE GUIDELINES:
Our bridge met the guidelines in all aspects which include bridge
material (solely basswood) , bridge weight ( <4.0 ounces), and bridge
capacity which exceeded the minimum requirement by an additional
75Ibs. The bridge trusses were constructed of individual hand cut
members in consideration of the direction of the grain of the wood tor
maximum force capabilities. It spanned the 30" gap with an extra inch
on either side. The bridge contained a flat, continuous, non-perforated
deck with a 4" width.
IMAGES OF FAILURE:
' (
POST-TESTING ANALYSIS:
In comparison of the bridge testing results and the preliminary design
predicted capacity, we exceeded the predicted capacity by 35Ibs;
and we exceeded the project requirement by 75Ibs. The location(s)
of failure were also successfully predicted by our team, due to the
focus on strength of primary truss assemblies and reduction of
cross member quantities and road deck thickness as a weight saving
measure; because in fact, the cross members were the precise
location of failure. These correct predictions are further verified
by the resulting broken bridge parts, where we had only two truss
member connections split out of both trusses, but nearly all of our
cross bracing separate from the trusses as well as a split down the
center of the road deck.
Thusly, the bridge broke under the twisting force of the Sib steel
bricks on the bridge due to the reduction of cross bracing. Further,
the discrepancies between the results and preliminary design
occurred as expected, and other than holding much more weight
than expected, our results went better than expected due to increased
overall capacity .
. IMPROVEMENTS:
Our design could have looked at a reduction of intermediate
members to 8 or less total bays, for example (instead of 10) to
reduce overall weight and maintain carrying capacity. We also could
have omitted at minimum, one 1/16"x1/4" basswood laminations
from our bottom custom laminated curved member to reduce weight
of basswood and glue in the assembly. This member proved to be
our strongest component and saw zero structural failure after testing
and therefore could have been reduced proportionally.
FURTHER IMPROVEMENTS:
Most importantly, we should have added more cross bracing and
angle bracing to add stability to the pair of trusses as a total bridge
assembly. We also (unintentionally) relied on the road deck to keep
the top of the trusses in parallel, since we omitted any bracing
between trusses directly below the road deck. In hindsight, we
should have added bracing in this location beneath the road deck
to ensure further structural integrity of our design. With the edits,
we believe the bridge would have performed even better than the
actual results, at a total capac ity of at or above double the anticipated
weight capac ity. (1 80+ lbs or more)