how much would a 4 x 6 rectangular tube deflec

huangyhg

how much would a 4" x 6" rectangular tube deflect >
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
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
gerald,
the deflection of a hss beam over 40鈥?using self weight only would be approx. 1.32鈥?br>
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/in虏
  stress limit = 50000.0 lb/in虏
  density = 0.283 lb/in鲁
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 in虏
  total wt. for 40鈥?section = 326 lbs
hope this helps
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
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
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.