Structure on top of trailer

[e104909]

Hi,

Imagine a structure with 4m column spacing in transverse direction pinned to 7m transport beam that is sitting directly on 2.5m wide spmt (SPMT - Self propelled modular trailer) deck without any fastenings.

Knowing that the structure is pinned, the bottom of the transport beam as also pinned, I have modelled the deck as a dummy element that will connect 3 bays of frame connected (as fixed) at the bottom.

There are two cases:
1. Assuming the dummy element is modelled with a dimension of 400mm x 600mm with the same stiffness as steel with zero density. The dummy element absorbs bending of significant value due to hogging and sagging plus axial force from the bottom of the structure's column

2. Assuming that the dummy element modelled with a dimension of 10mm x 10mm with the same stiffness as steel with zero density. For sure it will not take any significant force from the structure.

With these two assumptions, what do you think is more realistic? Is it more correct not to transfer any forces to dummy element and let the structure twist? which in reality might not possible to happen as it seems like the spmt deck is a very stiff element and the transport beam will not rotate either. The structures weight is 20tons

Hope to hear your thoughts

Cheers

Regards,
E104909

 

[DCBII]

I don't understand the problem. Some sketches would be helpful.

 

[e104909]

DCBII,

Here is a sample of 3D view in Staad pro. As you may have notice, the highlighted elements are dummy elements with zero density but with sufficient stiffness value and connected as fixed to the structure. These are the representation of trailer trucks. In this case, there are two trailers that serves as a support to the structure. There are "transport beams" where the structure is sitting in that is directly on top of the trailer.

Same material properties, but different sections of assumed trailers produces different behaviors. The structure is pinned to "transport beams". Take note that the transport beams has no connection to the trailers. Only friction would hold it in possible sliding.

Anyone who has experience in modular transport?

Thanks!



Regards,
E104909

 

[dhengr]

E104909:
I think DCBII meant a MEANINGFUL SKETCH, which shows you have a vague idea of what you are doing and what info. is needed to start discussing this problem. Put enough dimensions on it so we can imagine its size. From your word picture we could draw a dozen sketches, none of them representing the real load/structure. How is this structure supported in its final resting position or usage, what are the reactions? What are the loads on the six short columns just above the red thingies? What’s the total weight of the structure and where is its C.G. (about all three axes)? What are the column sizes and the sizes of the three lower gray beams (transverse beams?, transport beams?)? If we can believe that the six columns are spaced at about 13' o/c (your 4m o/c transverse), the two lines of columns are about 13' apart and the structure is about 30' long. I’d probably try to put this structure on one properly sized SPMT. You should be asking these questions of the people who are furnishing the SPMT’s, most of them know what they are doing, and can guide you based on the equipment they would be using.

 

[e104909]

DCBII & dhengr,

Thank you for your reply. Ok please see attached pdf. I have made a rough figurative sketches.

These past few days I thought I have finalized a concrete decision, unfortunately a specialist has commented to my drawing and say that the normal practice is that there is no connection between transport beam & structures column. They will just place a piece of "plywood" in this interface to improve friction forces that would be enough to counter any lateral movements..beyond that the transport contractor will provide lashings in which in our part is not a pre-engineered lashings. I believe transport contractors determine lashing requirements through stability checks

My problems grows to a point that obviously my structural assumption in this model is no longer applicable.

Any suggestions and any expert in this field?

Anyway, I dont believe transport contractors or SPMT experts have the same view as structural consultants..they didnt even produce any 3D model analysis to check the integrity of the structure..They are only responsible for the structures stability..beyond that I doubt it.

Regards,
E104909

 

[e104909]

I forgot to mention, total tonnage is approximately 5 tons.

This is a sample only..just to elaborate the problem.

I have platform of a total load of 30tons..bridge carrying equipments, pipes & cables with an eccentric COG which requires 2 line of SPMT's..but i believe the same approach should apply.

Thanks

Regards,
E104909

 

[JAE]

e104909,
The best solution is always to model the structure as close as possible to the behavior of the structural elements that you aren't modeling.
So the dummy, longitudinal members you have included in your model should behave similar to the SPMT's they are representing.

This would include mostly stiffness behavior under load (strength is a concern but not the key aspect of models).

So whatever type of dummy elements you include, they should mimic the vertical, lateral, longitudinal, and rotational resistance of the SPMT.

Sorry the above is not specific to your model - but a general statement - but as dhengr stated, we don't have all the facts.

 

[dhengr]

E104909:
Obviously, you need some engineering knowledge, judgement and experience to do this kind of work, but many of you guys are using Stadd Pro and other software, without that required experience, and thinking you are really solving a problem. What is your engineering background and experience? Are you a tech person/designer, CAD person, or an engineer? What type of industry? For a 5 or 10 or 30 ton load, half the time, they would crib these loads up on a low-boy trailer, or a few dollies and transfer beams, get a permit and drag it down the road. You are over thinking this problem, and they are used to moving 1500 ton loads on a few of these SPMT’s. Don’t change the damn sketch, just add the info. I asked for (legibly) and answer my questions, instead of changing the subject (sketch), and we’ll have something to talk about. Certainly, the movers should not damage your structure and to that extent you should/must interact with them, and they with you. But, they do so many of these types of loads that they just automatically know what to do on some issues where an inexperienced person thinks much engineering should be required. On your original sketch, answer my questions and think of (list for us) all the info. you think is needed for you, us and a mover to start to address and discuss this problem. You show them the load, your gray and blue sketch, without the red stuff that they will determine, with all the pertinent info. and they’ll tell you how they are going to move it. They don’t want to damage the load or drop it any more than you do, so give them the right info. and get out of their way. If you can put loads like you show on a single SPMT many of the twisting or torsional considerations on your load tend to go away. Furthermore, multiple SPMT’s will tend to self compensate for the twisting forces caused by elevation changes and slopes along the travel route.

 

[e104909]

JAE,

Thank you for the basic suggestion. I believe it is always the best approach specially if you are using softwares.

dhengr,

I believe you have not answered my query because you didn't really experienced the "fact" that JAE mentioned above. If you were saying that I am "over thinking" of the problem, maybe land transportation analysis is not needed at all. But to be bounded by basis of design, specifications set by clients, contractual issues and political agreements between clients and bosses, perhaps it is not really a good idea to put everything under the shade. We should know how to cover our ass out when the thing goes wrong. If transport contractor knows everything, then it would not be a usual case where consultants need to perform this analysis. Transport contractor cares to hold your structure along the way. But it is not their responsibility if your structure fails due to the reactions given by accelerations and other loadings. It is a negligence in your part.

Required experience is not just as it is. You cannot be a master of everything though. If you are a genius, I would still believe that you have the "basic knowledge" but on the spot you cannot really justify how it is really done in practice.

I dont know what industry you are specializing of but we all did go to school to learn the same way even if you would say you grew using slide rule. We all experience our first years as "JUNIOR ENGINEERS".

STAAD Pro is not a bad thing. It is you who should know how to use it. Even codes requires your part as an engineer to investigate it yourself and give your self the feeling of "satisfaction" as a professional. Companies is not really interested if you can do manual calculations alone. You should be able to convert your manual thing to the fastest way you know for us to submit our deliverable on time so we can convert this to cash. That is the point where intelligence has been defeated by just using common sense.







Regards,
E104909