Optimizing the depth of a structure

[SpecDor]

When designing a wall is there a fast way to know the most economical size of member to use. lets say a 30ft high by 30ft wide free standing wall. would 10", 12" or 16" vertical member produce the lightest structure. (assuming uniform windload only.

 

[rb1957]

it depends !

are you going to a fixed pitch (spacing) for the vertical members ? then smallest is going to be lightest (i think).

are you using the vertical members as the primary structure in resisting the bending load, like a cantilever ? then largest is going to be better (but using a truss and spreading out the base would be much more effective)

how much tailoring are you prepared to do ? (using the large section would be more efficient if you cut away chunks of the webs as you get away from the base would be more weight efficient)

 

[UcfSE]

You just have to calculate the different sizes you need and make sure you keep in mind everything else that will influence your design besides the wind load. Cheapest also depends on how you define it. The least material is one way. The least cost is another. You might be able to eliminate material and get weight down but incur so much fabrication cost that you make up for or exceed your savings.

Let's say this is a stud wall with sheathing. Your spacing needs to be a multiple of 8 for the sheathing to connect easily. Typical sheathing in the US comes in 4ftx8ft sheets. Using a large member at a large spacing might be good for the stud, but then maybe the sheathing will not work and you have to reduce spacing anyway. Using a small member with a small spacing might give you the least material weight but will require much more labor to install and you will have more connections to build. It all depends on what sizes and spacings you actually need. Generally, to save the labor costs, contractors I have talked to prefer to use the highest spacing they can even if it takes a larger or heavier gauge stud, or stronger wood species.

If this is a masonry wall, smaller is probably cheaper. If you start getting into very tight rebar spacing, or two bars per cell, it may or may not be cheaper to just increase the cmu thickness.