How much would a 4" x 6" Rectangular Tube Deflect > 2

[pipewelder1999]

I have a friend that builds boat dock and wants to know how much a 40' 4" x 6" x .120" Wall would sag over a 40' length under its own weight. These are used to attacc a recreational boat dock to the shore. If there is a formula I could use that I could understand I would use it but unfortunately I am not an engineer.

Both ends are free to rotate. The tubes will be installed with the 6" section vertical.

I have a theory that a round section would be more resistant to buckling from other forces. Is there any merit to this (Uneducated guess).

Thanks

Gerald Austin

Gerald Austin
Iuka, Mississippi

http://www.weldinginspectionsvcs.com

 

[desertfox]

Hi pipewelder

I calculate a deflection of 1.26" for the box section simply
supported at both ends over 40 ft.

the formula I used was as follows:-

max deflection at centre= 5*W*l^3/(384*E*I)


where W= WEIGHT OF BEAM 319LBS IN THIS CASE

E= MODULUS OF ELASTICITY OF STEEL= 30*10^6 psi

I= SECOND MOMENT OF AREA FOR THE BOX = 12.12in^4

l=LENGTH OF BEAM

regards desertfox

 

[ronstewart]

Gerald,

The deflection of a hss beam over 40’ using self weight only would be approx. 1.32”

The formula for calculating this is: (5/384) x (total wt. x length in inchs^3)/(modulus of elasticity x moment of inertia)


MATERIAL PROPERTIES
50W:
Modulus of elasticity = 29000000.0 lb/in²
Stress limit = 50000.0 lb/in²
Density = 0.283 lb/in³

CROSS-SECTION PROPERTIES
6x4x1/8 X:
Moment of inertia = 12.2 in^4
Top height = 3.0 in
Bottom height = 3.0 in
Area = 2.4 in²
Total wt. for 40’ section = 326 lbs

hope this helps

 

[pipewelder1999]

Thanks Desertfox and RonStewart. I had a spreadseet that a friend had made for me a couple of years ago but I wasn't sure whether in the "Load" cell what to put.

Thanks VERY much

Gerald


Gerald Austin
Iuka, Mississippi

http://www.weldinginspectionsvcs.com

 

[desertfox]

Hi pipewelder

Thankyou for your response its nice to know help is appreciated.

Have you or your friend considered strengthing this box section to reduce the deflection or are you going to a larger section beam?
A circular section would have equal strength in all directions unlike a rectangular shape which differing second
moments of area depending which way it is mounted relative to the load, a square section would possibly be another option but you would pay the penalty of gaining mass in the
horizontal direction to get the strength in the vertical plane.


regards desertfox



regards desertfox

 

[pipewelder1999]

Desertfox,

We did consider it. This is used basically to keep the dock from floating away on a lake. In addition I have drawn a walkway that will also aid the two "Spar poles".

I basically just do the drawings for him and weld but he sometimes asks my opinion and I thought a 6" Sch 40 Pipe would be great but the the rectangular sections are used on everything he builds so he wants to stay with that.

The walkway

http://www.weldinginspectionsvcs.com/walkway.htm

is entirely constructed from 2" x 3" rectangular tubing so it will do a pretty good job holdng since it is mounted on the same centers as the 2" x 6"s. We were mainly worried about how much sag would be there that could be seen.

Thanks again

Gerald Austin
Iuka, Mississippi

http://www.weldinginspectionsvcs.com