Multiple Layer Lattice Structure

[EmpiricalEngineer]

I'm needing some help to understand what is going on with some old calcs I'm reviewing. The structure is for an old timber roller coaster which is made up of hundreds of frames along the length of the track. The way I see it I think the wind loading Cf should be based on ASCE 7-10 29.5-2, with that wind pressure applied across the structure at every member. However in the older calcs I am seeing the Cf factor used as being 1.8 and for each layer in the depth of the bent frame, the Cf factor becomes Cf+Cf^0.8, Cf+Cf^0.8+Cf^0.8^2, Cf+Cf^0.8+Cf^0.8^2+Cf^0.8^3, etc. I would then assume that wind pressure would be divided up by the designated number of layers and then applied across that structure specifically. Very tedious and time-intensive, but the wind pressures I'm getting between the two methods would save 25% or so throughout the structure which is obviously something the original designer understood. Does anyone have any insight into where I can find the explanation of this Cf calculation?

Thanks!

 

[driftLimiter]

I'm not following the explanation of the old engineers calculations. Can you provide a typical sketch? Particularly this idea about 'each layer of the bent frame'.

My reading of ASCE 7 Ch 19 is that you take a single plane of the frame, calculate the ratio of solid area to gross area, then you get the Cf for that frame. Cf varies from 1.6 to 2.0 thats it.

 

[EmpiricalEngineer]

I would agree with you. From what I can understand they are using some kind of modified version of 29.5-3 but instead of the base number being 3.4 and 4 they’re using 1.8. So for a single layer bent their Cf is 1.8, for a 2 layer it’s 3.4, for 3 it’s 4.86, and so on using that equation I showed. I’ve just never approached wind load that way. I would assume as your Cf grows you reduce it back down by dividing out the layers to get your actual pressure to use on the frame.

 

[HTURKAK]

ASCE 7-10 Figure 29.5-3 is for force Coefficients ( Cf ) of Trussed Towers and suggests two formulas for square and triangle cross sections..
In your case, the structure is open frame with multiple bents .

The force coeff. is function of ( solidity ratio, frame spacing ratio and no. of frames ).

You may ( conservatively ) neglect the shielding effect and follow Section 29.5 or consider the shielding effect as discussed in your post.

In order to get the concept , i will suggest you to look

Wind Loads for Petrochemical and other industrial Facilities (prepared by ASCE Task Committee )