What is the strenght difference between double 4" steel channel welded together or a single 8" steel channel?

Question

The reason for my question is because I have a trailer frame 8'-6" x 36' being fabricated for me. The weight load will be approximately 10,000-15,000 lbs. The individual decided to use 4" steel channel 1/4" thick as the main beams and 3" channel for the cross members. Due to the extreme length of the trailer the individual has found a sagging issue within the middle of the main beams. He realizes now that he should have probably used 8" channel instead. Since the fabrication is so far ahead he concurred to double up with another 4" channel directly below the first to reinforce the strength due to it's length. I would like to know if this will make the frame even stronger that a 8" channel 1/4" thick.

Answer 1

The 8@11.5# light channel will be stronger than even two 4@7.25# heavy channels welded flange-to-flange to make an 8" deep member. The wider and thicker flanges of the 8" would be at optimum position to give the higher section modulus than the two 4" channel combination. This is because the flanges of the 4" channels are smaller than the 8'' and half of the 4" flanges are will be located in the centerline of the member where the bending stress will be the minimum.

Answer 2

Theoretically there will be slightly more material further from the central axis (of the loading) so it should be slightly stronger i.e. less bend and a higher overall failure load.

Back in the real world it will depend upon quality of welds, presumably he will weld them face to face i.e. you will end up with a 'H' shaped beam with the horizontal section twice as thick as the verticals, this can lead to corrosion issues as there will be a slight gap between the two faces that will be difficult to seal, could be a water trap etc..Also the welding procedure can interrupt the grains in the material which can accelerate corrosion (aided by fatigue loadings).

Make sure the beam is ultra clean before being welded and that the join is air tight thorough its life.

You would have been better off with the 8" in the first place.

In addition to the comments below I would like to stress that if the 'U' shaped channel sections are the same wall thickness as each other i.e. 1/4inch, then the centre of bending has changed, such that there is more material further from the centre of loading that there would be with a 1/4" wall thickness 8" channel, thus the actual depth of the beam would be 8 1/4" with the two 4" sections rather than 8" complete for the 8" channel.

Moreover, I do not believe that the said individual will weld flange to flange, more likely he will have artificially manufactured a box section, which would be a very poor solution to the problem.

Eitherway, the structure is of a magnitude that deserves some thought, for a person to realise at such an advance stage of the manufacture that the core design is inadequate shows the neglect that blights the profession of engineering.

Answer 3

Concur with the answer above that the corrosion may be an issue in the heat affected zone in the weld, however, welding the ends to seal the pipe at the ends. A good two part epoxy paint will help prevent corrosion, but it must be maintained.

My concern is the overall strength, making the I-beam from two channels isn't structurally sound because the web of the contributes to the strength.

I'd go to www.eng-tips.com and go to the structural form and ask if the welder or design engineer cannot demonstrate the strength is adequate.

Answer 4

i'm afraid that I have to agree with Ray, above.

The two 4" channels welded together will not be as strong as the 8" channel because the 8" channel has heavier flange sections.