Designing a lowboy trailer main frame

[Gungadinn]

Hi guys I have searched and cannot find exactly what I'm Looking for...

When designing a lowboy trailer main frame how do you compare mild steel to High Tensile Steel as in a frame for a lowboy trailer... is there any rule of thumb?

thanks in advance

 

[dhengr]

Gungadinn:
You don’t compare them, except to note that mild stl. has a lower Fy & Fu than the high strength stl. And, you will quick discover that you want the lt.wt.(light weight/dead load) of the trailer to be a min. so as to maximize the trailer’s cap’y., and still stay within a gross weight. Then you wrestle with the thought that these trailers take a lot of abuse, and the owner/driver always thinks that they should carry half again as much as the advertised cap’y. So, you don’t want to design them too lightly; they have to be tough and you do have to pay close attention to deflection. I have actually seen the underside dragging on the ground in some instances, not so good. Also, you don’t want vibration, particularly harmonic vertical motion to make it act like a rubber band, going down the road. The steel you select should be weldable, of course, and it should be tough in terms of fatigue resistance. Be careful to use good clean details in you fabrication and welding, and at transitions us fairly generous curvature as the members change shape and slope. Pay some attention to the difference in the way a large tracked machine loads the trailer, vs. how a large rubber tired vehicle loads at four more concentrated points; or how a steel drummed roller applies a line load across the trailer. The main frame consists of a center sill and two side sills, with regular cross members to tie them all together across the trailer. Then, there are some fairly drastic changes in shape and configuration at front, to the fifth wheel; and at the back over the rear axles, tires and suspension. Lowboys are really an interesting structure and design problem.

 

[Gungadinn]

dhengr
Sorry for the late reply, and thanks you for your info .... The main deck of the Lowboy is 20feet long....at the rear of deck there is a rear gooseneck 10 feet long about 6 inches higher than the deck height where the RSend suspension is mounted beneath the rear frame... at the front of the there is a gooseneck that is about 10 feet long 2 feet higher then the main deck that has a kingpin to connect to the towing vehicle...when calculating the bending moment of the main deck beam do I calculate as a fixed beam at each end or 2 rolling pivot points at both ends
thanks in advance .... cya .... Gungadin