This project was part of the 'Finite Element Application to structures' class, where it was given to model a Pedestrian bridge as an architect and also design it as a Structural Engineer. The criteria for the design of the bridge are as follows:
- Withstand wind speed of 100-120 mph
- Bear the weight of a Newfoundland puppy(don't be fooled by the term puppy! they weigh like horses)
- Located over a canyon, connecting the 2 sides with a span of 150 ft
- Be careful to prevent the lock-in phenomena
The conditions of the surrounding area is as shown in figure 1.1.( all dimensions are in ft)
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| Given conditions of project |
To catch the user's eye and to promote a sense of awe, a design idea of an arch-sparred cable bridge was put forward in our group of 4. The deck of the bridge is made up of a re-inforced concrete beam of width 12 in, which consists of W beams ( W6x8.5; W8x35 and W24x103). The supporting structure of the bridge is the arch which is a rectangular hollow tube, the diameter of the arch being 60ft. The arch is tied down to the bridge by 2 inch high strength tension cables running from the center of the arch to the center of the deck.
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| Model of the Cable Stayed Bridge in SAP2000 |
Loads Applied:
- Dead Load
Concrete Deck - 150 lb/ft3
Structural Steel - 490 lb/ft3
Railing - 95 plf ( pound per linear foot)
- Live Load
Pedestrian Loading - 149 lb/ft2
Equestrian Loading - 0.26 kip (applied to a 2x2 in area to check fro punching shear)
- Wind Load
The wind load acting is in 2 directions: the horizontal and the other in the vertical actin from under the bridge pushing it upwards
Horizontal - 61.36 mph
Vertical - 0.24 klf ( applied at quarter point of deck width)
Frequency Check:
As per AASHTO LRFD, the first fundamental frequency in the vertical direction,
f > 2.6 * ln (180/ dead load of deck)
so as to counteract any mode shapes generated from the repetitive forces on the bridge, like running or walking. This refers to the phenomenon of 'lock-in'. It is said to occur when the random frequencies of loading synchronize to match with the lateral frequency of the bridge. This point could cause high vibrations of the bridge, leading to failure. This condition has been seen in the Millennium Bridge, London, within the first two days of opening the bridge to the public.
Deflection Check:
The deflection limits of a pedestrian bridge as per AASHTO is that the bridge's lateral and vertical deflections should not exceed,
delta < (1/500) * span length
With the span length of 150ft, delta < 3.6 in
Actual deflections of the bridge:
Vertical : 2.75in; Lateral : 2.05in
So the bridge is safe from any abnormal deflections.
With the use of the software Revit ARCH, the idea of our bridge came to life as in the below pictures.
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| Final view of bridge (not to scale) |
The final requirement for this project was to create a video presentation good enough to sell the bridge to an architectural committee. The video presentation created is as follows:
