Suspension Bridge Design…
Bridge Tension-Compression Diagram
Sitting in another Structural Design Lecture we see the famous Huygen’s design method. It is said to be the most efficient manner in designing a suspension bridge. Analysing the design for any faults we think again…
San Francisco Bridge
The Huygen’s method for designing a suspension bridge is a simple method. It works on the principle that you use 2 pivot supports that you anchor and join using two tensile cables. The cables hang to form a natural parabola curve and then from the curve one hangs more tensile cables to shorten to span length and to carry the bridge. This is seemingly very efficient and logical, especially when one considers it has been in employ since the 17th century.
The problem with the design is that it works on the assumption that the cable will be working in tension only. Mathematical inspection ,however, reveals that the cable works in both tension and compression, a fact that could be predicted by the curve being a parabola and not a catenary, thus leaving room for improvement. It can be determined then that the amount of steel used in the cable design may be reduced by inserting a formation of truss bars known as the “Hencky Net”. This will then create a more efficient design and can decrease the material required by 0.3%.
While this number is not a drastic one it does lead to the questioning of design principles and efficiency in structural design as one could argue from an economic point of view that the Huygen’s method is more efficient as it lowers economic cost through less manufacturing costs. At this point one is forced to take a step back but in terms of true structural efficiency the Huygen’s suspension design principle are not the most efficient…
