I beam calculation 2

[starscreamer]

Hi all,

I am working on a flatbed trailer chassis design. Would anyone care to explain on how to manually calculate maximum stress in the beam under certain load?
Also, i did few simulation and it didn;t convince me at all. Is there any way to estimate or at least confirm whether the beam can sustain the desired load?

Thanks in advance.

Regards,

AFK

 

[desertfox]

try this link:-

http://www.efunda.com/formulae/solid_mechanics/beams/casestudy_beamtypes.cfm

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[starscreamer]

hi desertfox,

thanks for the reply. the link provided does not include any overhanging beam.
the beam i'm trying to design in considered overhanging with the fixture at the kingpin and at the center of the axle.

Regards,

AFK

 

[MikeHalloran]

You could analyze the ends as cantilever beams, with the additional complication that their 'fixed' ends are not actually fixed, but connected to the 'simply supported' portion between the kingpin and the axle center in such a way that the cantilevers' roots rotate with the ends of the central portion of the beam, so if you analyze it as three beams, the slopes must match at the supports, and the central beam has moments applied to the ends.

I think the load case you might find in a table is called a continuous beam over two supports at arbitrary positions with cantilevered ends, or something like that.

Of course in the typical flatbed semi-trailer, the beam is not of uniform depth from end to end, but is typically very shallow in the area forward and just aft of the kingpin, and somewhat less shallow in the area around the axle(s).

As for knowing when the beam is strong enough, if it reaches the yield point anywhere under any of the applied load sets, including dynamic loads, then "You're going to need a bigger beam".



Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

hi

Well perhaps it might of helped if we had of had a description of the beam setup at the start anyway try this:-

http://civilengineer.webinfolist.com/ohbcalc.htm

It sounds to me though that your beam might not be a standard case and actually be a statically indeterminate beam, why not gives us a sketch of the situation and we might help further.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[starscreamer]

hi desertfox,

i've attached a simple sketch of the beam. the beam will be fixed at the kingpin (1494mm from front) and at the axle center (9537mm from front). the beam web material is 4mm thick Weldox 700 and 10mm thick Weldox 700 top and bottom flange with width of 180mm.

Regards,

AFK

 

[kingnero]

Trailers typically appreciate a "not too stiff" design, as some flex in the frame absorbs the energy that otherwise would go towards the connections.

 

[starscreamer]

hi kingnero,

would you care to explain more on that?

 

[dicksewerrat]

Many trailers now are pre-stressed to form a hump in the trailer when not loaded. Go visit a couple of grain terminals to look at the trailers. Or a steel staging yard.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Leftwow]

Beams subjected to impact loads and cyclic impact loads at that, is not something a standard structural engineer would feel comfortable with. Seems like you would need to hire an engineering firm with some mechanical engineer that works with a lot of beam stresses.

 

[rb1957]

are there guidelines from your local MoT about load cases ?

are there requirements from your local MoT ? (crash loads, crash tests ?)

can you draw a moment diagram of the trailer ?

Why so much overhang ? or is there a 2nd container arrangement that uses more length ?

Have you looked at an existing trailer ? talked to trailer manufacturers ?

another day in paradise, or is paradise one day closer ?

 

[dhengr]

Ahmadfahmik:
Now all we need is a couple end views and a few cross sections through the trailer and we might have a vague idea what you think you are trying to do. Include dimensions, member sizes, design loads, etc. What is a Weldox 700? I get really worried that without a CAD system of some sort we would no longer be able to do even the simplest engineering problems any longer. Even with the CAD system, we certainly can’t describe them in engineering terms, to other engineers, and we seem to have little idea what info. is needed to have a meaningful engineering discussion. You might want to dig out your Statics, Engineering Mechanics, Strength of Materials and Basic Structural Analysis and Design text books and do a little self study before you put many of these on the road. Go ask your boss some of these questions, he should be guiding you on this, and he should know what you do and don’t know about engineering design, so he can keep you and the company out of trouble.

Your trailer structure is likely a grid work of beams; cross trailer members of several types, and longitudinal members, likely center sills and side sills, maybe some stringers. Your trailer structure is not fixed at the king pin, but rather is a cantilever, continuous over the cross trailer structure at the king pin. Nor is it fixed at (over) the rear axles or rear suspension, it’s a cantilever again, attached to a simple beam btwn. the king pin and the rear suspension. The trailer has a number of different max. load and stress conditions, depending upon the load it is carrying, and it can be loaded vertically, laterally, axially, and torsionaly. Then it should be designed, detailed and fabricated carefully, because it is a fatigue sensitive structure.

 

[rb1957]

I'd describe the trailer (as drawn) as a beam on three simple (pinned) supports.

Possibly (just possibly, given the simple question asked) the OP knows all this stuff and is really asking (given that we may have ESL at work here) "what should my allowable stress be ?"

another day in paradise, or is paradise one day closer ?

 

[starscreamer]

Hi all,

Thanks for the reply. First thing first, the beam i sketch is just a simplification of the real structure. the structure is designed with all the crossmember etc. but my main concern is will the main chassis (i-beam) be able to support the load.

Hi rb1957,
as for the manual calculation, correct me if i'm wrong, by drawing the moment diagram, then use the max moment to calculate max stress using formula sigma = My/I? the beam is designed to haul steel coil in roll form or placed on pallets. the load i mentioned earlier is just a simplification, as i think point load will stress the beam more compared to distributed load. and btw, it is paradise one day closer.

Hi dhengr,
thanks for the explanation in the first place. would you mind share your email so that i can discuss further with you?

 

[desertfox]

Hi ahmadfahmik

Yes if you really want that simplification I would treat the beam as simply supported and calculate the stress on the thinnest member section using the formula you quoted and based on the bending moment diagram.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[MikeHalloran]

Yes, flatbed trailers are typically precambered, and have been for some time.
Possible reasons include:
- Two troopers armed with a piece of string can determine that the beam is close to its design load.
- If the trailer gets swaybacked, the upper corners of cargo containers will bang on each other.

Mike Halloran
Pembroke Pines, FL, USA

 

[rb1957]

yes, I think a point load is conservative.

I think a trouble we're having is that there are a whole bunch of detail problems that can bite, and bite hard, if they're not considered.

for a start ... it sounds like you're working to a static design, but what about dynamic load effects, or fatigue loads ?
a static load should be an order of magnitude higher than your fatigue loads, but your fatigue allowables are much lower ...

another day in paradise, or is paradise one day closer ?

 

[dhengr]

Ahmadfahmik:
Don’t simplify things so drastically, you don’t do any of the rest of us any favors, you just leave us wondering, if you know what you are doing or not. If you have real drawings show them as pdf attachments, that info. is important to an experienced engineer trying to help answer your questions. Otherwise we just keep playing 20 questions, and never get to the important stuff. So, answer my/our questions because many of our answers might likely be different with different framing schemes, dimensions or different load magnitudes, positions, etc. And, read the meaningful posts twice for their full meaning, don’t just skim them and avoid the questions. Yes your trailer is basically a beam, but it is much more complicated than that simple statement. The details, the interaction btwn. various members, and good quality fabrication are every bit as important as checking the center sill design. You have to select your rear running gear, wheels, axles, suspension, etc. and design around that. You will have clearance issues over/around this rear running gear and the tractor area in the front. What size are your steel coils, how many on the trailer, what weight each, etc? Steel coils actually transport more securely when standing on their side in a saddle on the trailer or railcar. Try me at rwhaiatcomcastdotnet, no spaces in that string. Is this trailer for highway transport or just in-plant service? You should look for the design codes which govern this type of equipment in your area. States, national governments and manufacturing associations are a good place to start looking for codes and standards. Look at existing trailers to gain some understanding of what’s normally done and why. These flexible trailers are usually cambered (not pre-stressed), you build the camber in during fabrication. This is primarily for clearance reasons, but also because customers get really nervous if there is too much deflection under load. It looks over loaded, dangerous, over stressed. The camber always causes comment too, it looks funny, but its on an empty trailer so why worry..., the public thinks. Some loads are stiffer than these trailers, but not so with your steel coils or a flat stack of steel plate. Get your textbooks out, it sounds like you have a lot to learn. And, you should still be talking with your boss on this design, not just to people on the internet.

 

[oldrunner]

Start by finding the dead load reaction at the left end of just the trailer itself. This is actually not a simple beam as the depth of the supporting beam various from the shallow section, through the haunch-like transition section to the deepest section. So that alone requires some arithmetic. Then assume your load is a simple-beam in itself, calculate the reaction at each end. Your sketch indicates the center of the load outline - but be careful where the actual center of gravity is within your load outline. Now add the trailer dead load reaction and the left load reaction together. This is the reaction to the hauling unit. Multiply this times the 3000 distance. This is your bending moment at that point. (It's actually a little less if we deduct out the trailer bed section weight.) Now determine the section modulus of the truck bed support bed at the 3000 location. Make sure that all of your units are compatible, divide the bending moment by this section modulus and that will give you the static stress at that point - which is probably the controlling maximum stress for the unit. I assume that you know the material strength of the steel(i.e. the yield stress). For this estimate exercise, assume the allowable design stress to be of 60% of the yield and compare that to your calculated static stress.

(Note refer to a 2nd year (at least mine) strength of materials text to calculate the section properties of the supporting beam or beams. The supporting section may be a girder shape or a tube shape)

But this is just the start. To your static moment, increase this for impact - say 25% (I assume that you are going to travel at highway speeds). And because this hauling unit may be used daily, I would derate the allowable stress per the infinite life criteria (found in the latest AASHTO luminairies code). Now run your numbers again with the increased moment for impact and the decreased design load for fatigue. This should give you an idea the approximate capacity of the trailer structure. (That is, the final allowable design stress should be greater than the estimated actual stress)

But wait, we're not done. You still need to check the capacity of connection weld of the two flanges that meet at an angle (at the 3000 distance from the end). This you should probably have someone else do. We also haven't talked about shear through the web or the fillet welds on a welded girder, i.e., flange plates to webs. Hope they aren't skip welds.

Also, isn't there a rating for this trailer to start with? Or is this a home-made trailer cobbled from somebody's patterns and welded together by your welders - who might not be certified or the welding inspected?

Actually I had a firm to do this to me. They asked me to certify their trailer. This trailer was to carry a 50 ton load down our highways - which meant special permits. The steel was 100 ksi yield and the webs had numerous cutouts to lighten the dead load. Our fee was rather high and they rejected us. They built the trailer and load tested it. Broke in half.

Maybe you shouldn't take the responsibility of hazarding a guess to the capacity. Also slow down for curves and pay attention to banked roadways.

 

[oldrunner]

Just a little important correction to my earlier comment: "The decreased design load for fatigue." should read "The decreased allowable design stresses for fatigue."

Sorry