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Wind Loads from Varying Directions
3

Wind Loads from Varying Directions

Wind Loads from Varying Directions

(OP)
I'm a relatively new user to RISA 3D and am having some trouble coming up with the simplest way to apply loads to a structure.

Structure in question is similar to the attached. There are also discrete loads attached to the vertical pipes. In this case, the structure and its discrete loads are symmetrical on 120° increments but that's not always the case. I'd like to apply wind loads in 15° increments for 120° around the structure to capture all potential load cases. This is probably excessive but I'm really using this specific case as an exercise to better learn RISA.

How I approached this was to build 1 of the 3 sections, apply the discrete loads as point loads, and apply distributed loads to all the members. As the wind rotated from 0 to 120 degrees, I broke the loads into their X and Z components. However, when I copy/rotated this thing to grab all 3 segments, it became unwieldy. When the loads copied over, they stayed the same in magnitude which wouldn't be accurate as the projected areas changed relative to the direction of the wind.

Looking thru RISA help, I feel like there is some way to approach this that is better but I can't figure out what that might be. I thought I had a break-thru this morning when I read about the distributed loads as pressure on a projected area. But I don't think that works if I try to break that down into components for my load cases. (Say a member that is 45° to perpendicular with the wind parallel to the member - component pressure loads would still add a significant axial load to the member)

My next thought was to use the pressures but save-as and rotate the model for each different wind case.

And again, I realize I'm likely over-cooking this specific analysis and am really looking for input on how some of you would approach this within the context of load application within RISA.

Thoughts on this scenario would be greatly appreciated.

RE: Wind Loads from Varying Directions

2
Most loads in RISA are either applied relative to a local axis or the global axes.

Global Axes:
Back in the day, I did something very similar to this in a quick and somewhat dirty way. I created multiple copies of the structure. Each one rotated 15 degrees relative to the one before. Saved all in one file. Then I applied the same wind load based on global axes to the structure. A little messy, but it accomplished what you described.

Local Axes:
If you used "member area loads" you can apply a pressure load perpendicular to the local plane defined by the 3 or 4 points of the area load. You have to create a bunch of different joints solely for the application of these loads, of course.

Alternatively, you break the joint loads down into different X, Y and Z components for each 15 degree rotation. This should be easy to do by copying and pasting back and forth from Excel and such. Though I'm not a huge fan of this because it's a lot harder to visually confirm your loading in the program.

Ideally, RISA would add a feature where you could rotate a joint load relative to the global coordinates so that it would still show up as a single load (but at an angle) instead of forcing it to display as load components.

RE: Wind Loads from Varying Directions

(OP)
Thanks Josh - appreciate the input. I guess I was hoping there was some way to rotate the loads as you described at the end but apparently not.

I think I'll mess around with rotating the whole structure and keeping the loads in one axis. As you noted, breaking into components makes is hard to track and visualize.

I also believe that applying the loads on the members as "projected loads" or "pressure loads" in the global axis will properly scale the loads as the member length changes with rotation relative to the global axis wind load direction. Does that seem correct? If so, that would save a lot of spreadsheet math.

RE: Wind Loads from Varying Directions

(OP)
So I've been working with this and went about modeling it where I had multiple copies of the same structure in the model which were rotated as needed. This was much simpler as I no longer had wind loads in two axes.

But now I'm down to trying to extract information for my reactions and connection design and with all the members it is again unwieldy.

So a more simple question: Is there a way for the spreadsheet reports to only include 'selected members' that are selected in the models? Or is this part of the fun of deciphering the data?

RE: Wind Loads from Varying Directions

For the loads on the vertical pipes I would start by calculating the wind load in the Z and X direction. Then I would create a basic load case for each Z and X direction. You can then reduce the Z and X direction wind loads, for the specific wind direction, by changing the factors in the load combination spreadsheet. So basically the equation to get your load combinations would be the following:

1.2D+1.0W=1.2D+1.0*COS(ANGLE)*WLZ+1.0*SIN(ANGLE)*WLX

RE: Wind Loads from Varying Directions

(OP)
Thank you on the 'exclude' button - exactly what I was looking for.

Regarding the loading - the vertical pipes are the easy one. It's all the other parts and additional discrete loads that are difficult to track once it is copy-rotated on a 120° pattern. X and Z get confusing and you instantly triple everything.

If I'm working it correctly, the distributed load acting as a pressure over projected area is doing what I need it to do for the member loading.

Thanks again for your input - I'll keep learning.

RE: Wind Loads from Varying Directions

I would do the same thing for the loads on the members that I did for the vertical pipes.

Assuming you are following the equation in section 2.6.9.2 of TIA: EPAa=Ka*(EPAn*cos^2(angle)+EPAt*sin^2(angle))

I would calculate the uniform distributed load in the Z and X direction for each member. Then using the load combination spreadsheet to reduce the Z and X direction wind loads, for the desired wind direction. The resultant uniform distributed load, from the Z and X direction, should be the same as the uniform distributed load using the equation in section 2.6.9.2.

For RISA I would use PX and PZ and not SX and SZ. That is because the pressure (SX and SZ) won't take into account the added projected area when the member has ice on it. You have to do that manually and calculate the uniform load when it has ice on it. But if you are working in a non ice area then it doesn't really matter.

RE: Wind Loads from Varying Directions

(OP)
No ice in my consideration - haven't jumped to that issue yet.

I initially approached this problem as you described and was planning on using load factors in the LC. But I have an issue with this approach. Say you have a horizontal member (3" OD pipe for arguments sake) that is inclined 45° to the Z axis. It has an equal projected area and therefore distributed load in the X and Z direction. Now consider wind inclined 45° to the Z axis, parallel to the pipe. Forces normal to the pipe from each component cancel as expected, but you end up with an axial force in the pipe that is much larger than just the exposed end of the pipe. That's what led me down the rabbit hole of the pressure on exposed surface.

Another example - say you have an appurtenance with a front load of 20# and a side load of 5#. For wind at 45°, your approach with the vectors would yield a load of 14.58#. The TIA formula would yield a load of 12.5#.

I've been too deep in this now today an my brain hurts. Gonna go write do some brainless work for a bit.

I do appreciate your input and this discussion.

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