Total Member Length (ft): This is directly proportionate to material cost
Maximum force in a member (kips): design demand
Most Important Tradeoffs to Consider
A larger truss will decrease structure forces, but increase total member length and therefore total cost
Design Decision 2: Structural Bay Size in Office Building
Design Variables
Bay Width (ft)
Bay depth (ft)
Evaluators
Structural Steel per floor
Total Floor Area
Most Important Tradeoffs to Consider
Larger bays increase net area which therefore increase building value, but also increases structural costs.
Design Decision 3: Number of Windows v Energy Cost
Design Variables
Size of windows
Size of walls
Evaluators
Annual Energy cost
Level of natural light measured
Most Important Tradeoffs to Consider
More windows allows more natural light, but increases solar heat gain and drives up prices from construction to energy bill. a good balance of glazing ratio should give an optimal performance here.
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Step 2 - Generative Design Study
The goal is to determine the optimal truss configuration for a 50-ft span bridge that balances material cost and structural performance. What combination of height and panel number minimizes total length and maximum force in a member.
The base truss model was designed in grasshopper as shown below. The bridge was designed with a constant 50ft span, all members summed was the output of total member length, and maximum diagonal force was calculated by this formula, standard for a warren truss: F = (wL/2) × sqrt((L/2N)² + H²) / H.
Design Variables:
Truss Height (H) from 4-12ft
Number of Panels (N) from 3-10
Constants:
50 ft Bridge Span
Uniform load of 1k/ft
Evaluators:
Total Member Length (ft) - minimizing this would lead to minimizing material costs
Maximum diagonal force (kips) - minimizing requires larger members
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Step 3 - Generative Design Study Results
Preliminary Design of the Truss for visualization purposes:
Galapagos Solvers:
Minimum Length Solver: N=3ft (min slider), H=4ft (min slider), L = 148.04ft, F = 57.77 kips
Here is plotted the Diagonal Member Force v the Total Member Length for different numbers of panels at different height values for a fixed 50ft span truss bridge. What is shown reinforces the results from Galapagos; The lowest value of Total member length is also the highest value for that diagonal force and the lowest value for chord force corresponds to the highest value of total member length. This is a great representation of the tradeoffs needed to be made during structural design: the bridge with the lowest forces also cost more and the bridge tat is cheapest will have more forces pass through it. There is technically no right answer and a design decision needs to be made based on what is needed. There is however a range of points that compromises well between the both of them between around F=28-38kip. & L=160-190ft.
In this case, what I would do is find out what is the diagonal force I need to design for and find a design that works for that. If the chord force can be 100+ kip, then I don’t necessarily have to worry about being in that most efficient range since I can save a lot on member cost. If I do need to be in that F=28-38 kip, then I noticed that different amount of bays and height combinations overlap and I would see what would be the most visually appealing of the options hovering around the same value.
