Calculating Aluminum I-Beam Strength

[deendick]

I have a 24,000 pound boat being lifted by cables, bow and stern, attached to two 6inX10inX16ft aluminum I-Beams. Top and bottom flange ½ inch thick, 5/16 inch thick web.
The beams flex approximately 1¼ inches from center to ends. Is this deflection acceptable/dangerous? Is this beam too small?

 

[40818]

Deendick,
In order to ascertain whether its a problem you need to supply data such as material type (i.e grade/spec of aluminium), whether the load is applied gradually or immendiately, what safety factor are you working to, what failure mechanism are you considering. All these things need to be known.

I have just did some quick math in my head, and i reckon your stresses are around 18000psi. So you should be ok from a pure tensile/compressive stress point of view with a safety factor of say 2 (till yield). I would want a much greater safety factor though (probably 8, so your no where near that).

 

[kenvlach]

The only aluminum structural material available in the US (that I've ever seen) is 6061-T6. Properties:

http://www.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061T6

Dimensions & weights of aluminum I-beams (extruded) including 10x6 (note: web given as 0.25", flange as 0.41"):

http://schupanalum.thomasnet.com/vi...inum-association-6061-t6-extruded-?&forward=1

Note: Aluminum beams are also available in narrow & wide flange dimensions:

http://schupanalum.thomasnet.com/category/extruded-shapes-structurals

 

[deendick]

Thank you 40818 and kenvlach for your response. It is greatly appreciated.
To 40818...I do not know the material type/spec of the aluminum beam. Since the lift was manufactured in Florida I would have to assume it is the 6061-T6. 24,000 pounds would be the extreme weight of the boat with fully loaded water and fuel tanks and supplies. This beam is used in a boat lift, so the load is applied gradually as the boat is lifted, but then hangs over the water until it is lowered again. In the winter this could be as long as 5 to 6 months. There are no safety factors built into the lift...if the beam fails, the boat falls back into the water.
Your quick math in your head, (remarkable), resulting in a safety factor of 2, makes me a little more comfortable, but I also would like more wiggle room.
To kenvlach...the addition of the web links was a nice touch. I have contacted Schupan & Sons and asked for their recomendation for beam size.