Lateral deflection of a semi gantry crane runway beam and column

[Chandan321]

Hi All,
I am designing the upper support of a semi-gantry crane, with the following details:

1. Total wheel loads along with impact factor = 650 kips

2. The crane length = 135 feet (centerline of upper rail to centerline of lower rail)

3. Height of upper rail = 84 feet from top of pile cap

4a. I am trying to support the upper end of the semi-gantry crane by a corbel and a 6' x 4' concrete column, supported on pile cap

4b The columns are 37'4" apart along the runway beam length.

5. Seismic factor = 0.2

6. It seems that the lateral deflection of the column is 9", which I guess is unacceptable

7. I tried 8' x 8' column, but the deflection is 2.5"

8. I also tried a truss system with 2 W36 columns and laterally and longitudinally well braced, but the deflection is 3.5".

Are the lateral deflections OK, for the crane to not jump off the rails. If not, how can I limit it to less than 1", which my lead wants me to do.

Thanks as always to all of you.

 

[JedClampett]

I would talk with the Gantry Crane manufacturer. If I understand you, you're trying to reduce a lateral deflection of a 84 ft. tall cantilever member to .001 times H. That's going to be extremely difficult.
Plus, has you supervisor considered that no matter how stiff you make the column or truss, any angular deflection in the pile cap and piles is going to control?
If you need to restrict deflection, deeper is better. A deep truss is going to give you the least deflection, but it might start looking funny.

 

[msquared48]

Any chance of installing a tieback member, W shape or otherwise, at maybe 2/3 of the height of the column - around the 55 to 60 foot mark to limit the deflection?

Mike McCann
MMC Engineering

 

[Chandan321]

Thanks for your responses.

JedClampett, can you explain a bit more by "any angular deflection in the pile cap and piles is going to control".

msquared48, I forgot to mention that the W36 columns are tied at 6 places along the height. The tie beams connect the columns 37'4" apart.

Also, I understand that no matter how tall the crane is or how long the crane is, if the actual deflection is say a couple of inches, there is a chance of the wheels actually jumping off the rails. Do you think my understanding is correct.

Also, which of the two options (concrete column, or braced truss system) do you think is the better one structurally.

Thanks again to all of you for your time.

 

[slickdeals]

Jedclampett:
You bring up an interesting point about the tip deflections being affected by the rotation of the pile cap.

Are there specified procedures that structural engineers can use to compute the rotation? I guess we can use the axial stiffness of the piles and model them under pile cap and check rotations, but won't the axial stiffness of a pile in compression (end bearing + skin friction) be different than in tension (skin friction only). Any thoughts?

 

[JedClampett]

We normally try to not calculate the rotation of a support like that. We try to design it to just support the loads in a conservative way. It's unusual that the support rotation would affect anything. But if you're trying to limit deflection at a location 84 ft. up, you can't say that doesn't impact it. An angle of .1 degree causes a large movement.
I would estimate the stresses in the piles due to the couple forces. Assume the cap as very stiff. Then calculate the shortening of the piles in compression and the lengthening of the piles in tension over some distance, say one-half its length and use trig to calculate the angle. It's not worth trying to be too accurate.

 

[hokie66]

Jed is correct about the rotation at the base of the support, and this problem is very difficult to solve without a big lever arm. I would try to use Mike's suggestion of bracing the column rather than trying to make it stiff enough not to deflect.

The other issue, as Jed said, is the gantry crane frame itself. The stiffer the 84' high x 135' wide L frame is made, the less problem with the support column.