load capacity of aluminum tube

huangyhg

load capacity of aluminum tube
i am considering building some aluminum ramps, that would be 5ft long.  i was planning on making them out of 1 x 2 x .125 6061-t6 tube.    each ramp would have three tubes running the 5ft length and have crossmembers joining them and then a .125 piece of floor plate over them.
my questiion is what is the capacity of each tube or even better of the three combined.  the ramps would be supported at each edge so it would be a 5ft span.
long time ago i had found some charts that had similar information but for the life of me i cant find them.
thanks alot.
if no chart exists and their is a formula that will do the job.
will the 1" side or the 2" side be turned up?  what is the spacing between the 3   
the 2 inch side would be vertical and the width would be 12 inches so 5.5" from center to center  and they would be at a 30 degree angle from the ground
thanks
just buy the pickup ramps.  tractor supply has a nice selection.
mike halloran
pembroke pines, fl, usa
a 2"x1"x0.125" tube has a bending modulus of 0.23in^3.
fcy for 6061t6 is 36ksi
allowable moment is 8238 in.lbs.
the tube is 60" long, assume a point load in the middle
allowable shear load is 8238*4/60 = 549 lbs
if you use a safety factor of 3, then the allowable load for your assembly of 3 tubes would be 549 lbs; this assumes all tubes are equally loaded which is not obviously the case.
this assumes your floor plate is ineffective.  its effectivity depends on how well you connect it to the tubes (weld?, 'cause you're using 6061).
you also need to think about the floor plate itself.  it distributes the load to the tubes.  say a load of 500 lbs is applied at two points (2*250 lbs) on the ramp (a small trolley) say 6" apart, centered on the ramp.  the outer tubes react 1/2, 125 lbs; maximum moment is 125*3" = 375in.lbs; stress is 375*6*64/w = 144000/w psi ... so if this floor plate is steel, you need to have 2" effective width (with no safety factor) ... ha hummm a bit dodgy, but then your cross   
thanks so much for your help.
just for future reference how did you get the 8238in.lbs from the information you had and also is their a chart or list of different tube sizes and amterials.
thanks
just reverse the bending stress equation ...
stress = moment/section modulus
so allowable moment = allowable stress*section modulus
=36000*0.23
kanoa, are you an engineer?  
if you are welding the 6061-t6 alloy you must use the as welded condition or reheat treat (postweld solution heat treatment and artificial aging in order to restore some strength and return the structure to the -t6 temper).  the 6061-t6 base materials, as purchased, has a typical tensile strength of 45 ksi before welding. the aws d1.2 structural code has recognized the metallurgical changes that occur to this base material from the exposure to heat during arc welding, and consequently, requires a minimum tensile strength of 24 ksi. to comply with aws both weld size/shape and interpass condition must be met.   in the as welded condition 6061-t6 with er5356 rod we often use 19 ksi yield strength, with er4043 rod we use 18 ksi yield.
the welding rod must be selected with heat treat plans (some fillers undergo undesirable changes if subjected to heat treatment) and final finish in mind (some fillers will change color during anodizing).
cheers