Seismic Diagonal Load?

[SlideRuleEra]

I'm designing elevated structural steel support for a set of four ash hoppers at an existing electric power plant. Each hopper is essentially a tank with four short legs. Each leg has a base plate that will be anchored to the new steel members.

The hopper manufacturer has given the table of loads (dead, live, tank contents, seismic) at each base plate.
My question concerns the seismic loading, three load are given:
1. "Seismic Normal"
2. "Seismic Shear"
3. "Seismic Diagonal"
I have not encounterd the term "Seismic Diagonal" before. Insight into to this term will be appreciated.

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[jjeng2]

Is there cross bracing or X bracing between the legs. It could be an axial force in a brace? What about a 45 degree horizontal load? I think that would be the same as the shear load but Im just brainstorming.

 

[JStephen]

Is it just the vector sum of the other two forces?

Anyway, don't be afraid to just call the designer and ask- no reason to guess on something like this.

 

[JStephen]

Another possibility- if you use I-beams as legs for a structure, oriented either radially or tangentially, and then apply a lateral load to the structure, with the legs pinned at the base, you will generate reactions at the base that are at right angles to the load direction. Maybe this is what they're getting at.

 

[TFL]

maybe something like quartering loads? just a gurss

 

[JAE]

Wouldn't this just be three different seismic load directions? The Normal load could be seismic oriented in some perpendicular fashion, the Shear could the direction at 90 degrees from the Normal, and the Diagonal would be seismic at 45 degrees from the principal axes (in plan) of the hopper?

 

[SlideRuleEra]

Thanks for the comments so far. Here are the magnitudes of the loads:
Seismic Normal: plus/minus 8 kips
Seismic Shear: plus/minus 3 kips
Seismic Diagonal: plus/minus 11 kips

For now, I have been assuming the following:
"Normal" is vertical.
"Shear" is horizontal (from any direction).

Had considered "Diagonal" as being the result of vector addition, but that does not seem to make sense with the numbers presented.

JStephen - Will call the vendor, if necesssary, however it is still early in the project and was assuming that this "Diagonal" force was a "gap" in my knowledge. (Didn't want risk embarrassment if this is an "easy" question.)

http://www.SlideRuleEra.net

 

[jjeng2]

SlideRuleEra,
Let us know when you find out. We can see if anyone was close and we will know if we ever come across it.

 

[crossframe]

Before SlideRuleEra spills the answer, I wanted to get my guess in.

The magnitude of the loads makes me think it's something similar to AASHTO's seismic load cases (1.0Longit. + 0.3Trans. & 1.0Trans. + 0.3Longit.) to account for forces "diagonal" to the structure.

 

[pba]

Here's my guess.
Normal is vertical.
Shear is horizonal.
Diagonal is exactly that, the force assuming acceleration across the diagonal of the hopper such that opposite corner columns provide the push pull effect. - 8xroot(2) - Its also a vertical load.

 

[SlideRuleEra]

Thanks to all for your input on this subject. I had hopes of finding out the answer, but the truth is that I did not get the opportunity - my proposal was not accepted - insufficient errors & ommissions insurance. Oh, well, lesson learned... "back to the drawing board".

http://www.SlideRuleEra.net