Extra frame support 2

[b2drawingboard]

Hi , I am building a mobile laboratory that is proving to be heavier then I originally anticipated. The unit is 12' x 60' with two 14/30 I-beams for the main support of the structure and I have two 25k axles underneath. I have a length of about 30' that I would like to add some extra load support too in front of the axles but don't know how to determine the amount of support I need to prevent the frame from flexing. The main flooring is designed like a fifth wheel heavy equipment hauler. Any help is greatly appreciated.

 

[r13]

It sounds like a hauler frame. You need to provide the elevation view to show the distance of the axles and the 2 W14x30 beams. Also the weight it needs to support. When you say you have 2 - 25kips axle, is that the net support capacity with safety factor has already built in? Did you consider impact in capacity determination?

 

[Rabbit12]

What is your definition of flexing? How much is too much? What did you use to design the first two beams?

Is it just me or are the DIY posters coming out of the woodworks lately?

 

[BUGGAR]

I think he means lateral flexing when all those wheels go around a curve.

 

[dhengr]

Rabbit12:
DIY’ers? They can’t even describe a brick, let alone know what to do with it, but if they want to design a brick or think they need a brick, they Google ‘engineering’ and E-Tips is popular enough so it pops up. Then, we keep giving away free engineering advice to people who probably misuse it, for lack of understanding what we are talking about. We keep playing 20 questions with them, when they start by framing a trailer with a couple of W14x30’s, their first mistake. And, we, for lack of real productive work effort for the guy who’s actually paying us for our time, keep right on participating. Have you noticed how many of the smart, experienced, long term contributors to the various forums have just kinda quit playing this game? E-Tips really isn’t a site for engineers interacting and communications with other engineers any longer, for the betterment of the engineering community, it has just become a cash-cow for the owners, as long as we (the knowledgeable contributors) keep feeding it. It is really riding on its history, and going downhill pretty fast. Soon, we will all just be ‘Maker’s of Projects,’ lots of members with crazy ideals and little knowledge, which makes for ad dollars. Who cares?

 

[r13]

We are allowed to "red flag" a post the is deemed inappropriate for this forum, don't we? I tend to offer ideas to the people, who have the basic, but need guidance and safety precautions. If the DIYer can't answer the posted basic questions, the discussion is end. If he can, then I'll offer help for him to think about, and let him do his own work with plenty of safety factors build in. This is my stance, everybody has his/her own opinions.

 

[b2drawingboard]

I do appreciate everyone's view point and I completely understand the frustration that's fueling some of the unhelpful comments. This unit is my project However I did not attempt to build this myself. I contracted out to a well known company and after pulling a string along the I-Beams I was able to figure out the frame was deflecting 5/16" . I was reaching out to what I thought would be professional help from some well grounded gentleman to figure out the best way to better reinforce the frame. I do appreciate any helpful feedback.

 

[CivilSigma]

Definitely get your money back from that company if you feel that their design is unsafe ...

 

[dauwerda]

5/16" deflection sounds very small for a 60' long trailer.

 

[b2drawingboard]

dauwerda , I do agree with you on that point and I really don't think its a safety issue. The unit has passed its inspection for structural integrity . The customer has requested there to be no more the 1/16" of deflection . In my opinion that is a bit extreme given the size of this beast. My plan was to add some more support to reduce the amount of deflection as much as possible and live with the result. The main reason I am reaching out is I am trying to decide whether to use 6" channel or rectangle tubing. At this point I think I will go with the tubing and secure under the length of the I-beams even if it's over kill. I will be well inside my maximum load range when using the added steel unless there is another variable that I'm not aware of.

 

[jayrod12]

I would be considering either HSS or angles nested into the corners and welded all up so it stay symmetrical. Something like the attached sketch. A W14x30 isn't incredibly stiff, so adding 4 - L4x4x3/8" in the orientation shown would probably do wonders for stiffening it up even more.

Is that deflection measured under full load, or just under self weight? Besides "the client", what is driving the 1/8" requirement. Keeping it to 1/8" in 60 ft is a battle just for self weight, not to mention borderline impossible under full load.

 

[dauwerda]

Is this a flatbed trailer or is it a box trailer? It seems to me the only way to keep deflections that low would for it to be a box trailer and utilize the sides as deep trusses

 

[Tmoose]

"The customer has requested there to be no more the 1/16" of deflection ."

Was that deflection limit mentioned in the RFQ, or just added in a phone call yesterday afternoon ?
As others said, how are you defining deflection ?

.032" in 720" is sort of like span/11,000. An order of magnitude less compared to floor construction to prevent cracking drywall.

 

[b2drawingboard]

jayrod12 , I only have access to the lower half portion of the I-Beam at this point But I could definitely add the L4X4X3/8" there at the very least.

 

[r13]

If you do have free vertical spacing available, add any shape that increase the depth of the beam will help. Since for simply supported beam with uniform load (w in plf), the deflection (in inch) is 5*w*L⁴*12³/384*E*I, you can set it equal to the desirable deflection 1/16", and rearrange the equation/terms to find required I (moment of inertia, in⁴), with L (beam length in ft), and E (elastic modulus in psi). The trick is finding the moment of inertia (I) for the built-up section. You maybe able to do it yourself, or get help from someone. The ideal shape will be adding a HSS with horizontal dimension wider than W14 for ease of welding, yet not adding too much of weight.

 

[dauwerda]

As you can see in the equation posted by retired13, the effect of the moment of inertia (I) is linear in regards to the amount of deflection. So, if you currently have 5/16" of deflection and you are trying to reduce this to 1/16" of deflection, you will need to increase your moment of inertia by 5 times. Adding a few angles inside the flanges of your existing beam isn't going to do this.

 

[r13]

I was about to sound the same alarm, dauwerda is correct. You may have to add another beam. Can you post your framing plan and elevation view? Just simple sketches will do.

 

[jayrod12]

That's a good point dauwerda, and I agree. That being said 1/16" in 60 feet is unreasonable anyway. Even the beam weight deflection over that span is an inch.

 

[dauwerda]

Agree jayrod12, we obviously don't have the full story here. It is a trailer so its not a 60' simply supported beam, depending on where the axle(s) are located I'm sure its more of a simple beam with cantilever.

 

[b2drawingboard]

Yes I will get a drawing asap.