Transportation of piperack modules

[rtmote]

I have client specs that call for acceleration forces for transporting piperack modules to be 0.3g longitudinally and laterally and 1.5g vertically.
We have two transport options, Scheuerle (continuous low level loader) and dolly (independent front and rear axles, tied through the structural system being transported).
Unfortunately, the spec does not tell you how to do the analysis.
On the dolly system, I have engineers calculating the extreme longitudinal force, statically, on 30T (metric)load being 9T horizontally. This seems overly optimistic.
I believe the 0.3g is the variation during transportation. The problem must be accelerating and decelerating conditions and the weight should be thrown longitudinally, meaning the design force should be (1.0 + 0.3)g giving 39T.
On the scheuerle system it is not critical as the module is tied down everywhere and the transport base provides the stiffness for transportation.
Any suggestions out there ?

 

[civilperson]

If the module were dragged on a surface giving a high coefficient of friction then perhaps the longitudinal force experienced might approach 0.3G. If the the axles, tires and brakes are of normal quality, then the 0.3G is very conservative. If forces sufficient to make 1.0G were applied, the the acceleration would be 9.8 m/sec squared, (enough to get to 100 MPH in less than 6 seconds).

 

[rtmote]

The problem I have is agreeing that 0.3g is the design force, without factors. How do I get to design forces? Or how do I apply the 0.3g for design?
I have transported tall vessels on saddles, using dollies, but 36 m length piperacks of statically indeterminate stiffness?
Should 0.3g be the applied force in a single bracing or assumed distributed throughout. At least an estimate of stiffness of the system connecting the front to the rear wheels is required. The weight of the rear wheel system must also be included too.

 

[civilperson]

Not the weight, rather the drag force of the dolly/rear wheel system is applied longitudinally. What are you designing? A frame for connecting the rack to the wheels?

 

[rtmote]

It is a 36 m (120 ft)long cable tray trussed module 6.7m wide and 2.6 m high, 30MT in weight, suppported at two locations 24 m apart (so 6m overhangs either end). The module suports are W310x60 columns 1.2 m high.

This module on four columns is directly connecting the front to the rear wheels.

 

[civilperson]

Assuming my understanding of the components and construction, there are two columns at the back on the rear dolly and two at the front dolly. Use horizontal loads applied to the bottom of these columns simultaneously with the vertical load. One column base would experience: 1.5x7.5= 11.25 MT upwards, 2.25 MT longitudinally and 2.25 MT transversely. These loads reflect the static equivalent of a moving system. Check the various parts for stress and add braces/ increase sizes as necessary. (Axial stress 2.1 ksi, X axis bending stress 4.5 ksi, y axis bending stress 21.3 ksi in the support column assuming a cantilever)

 

[J1D]

rtmote

Client specs do not generally tell you how to do the analysis. The engineer designing the steel module should find proper design specs if the design company has no design guideline. Longitudinal force of “(1.0 + 0.3)g giving 39T” seems overkill. 0.25g is the force we use as the horizontal force due to braking or acceleration (in longitudinal and transverse directions, but they don’t happen concurrently, they are combined to 1.5g gravity load separately). Sometimes, most critical forces occur in structural members during transportation.

In analysis of transportation (On the dolly system), the front (and rear) columns may need to be designed to take the whole longitudinal force.

 

[J1D]

To apply the 0.3g horizontal load. You can 1)apply 0.3g structural weight through a 0.3 gravity factor in an analysis program (in horizontal direction); 2)apply 30% of the cable tray load on the support beams in the horizontal direction as well.

 

[rtmote]

Thanks for the input, I think you answer my questions on the global level but I am concerned about detecting weaknesses in design, particularly for the dolly system. I am inclined to check the frequency and stiffness for the dollied system, as the cable tray modules are much lighter and less stiff than the heavier and deeper piperack modules.

 

[J1D]

The cable tray module cross-section is about 22ft wide by 8’-6” high, not necessarily the model is less stiff than a higher piperack module. Particularly in transportation , with the supports 80ft apart, the module must be framed with vertical braces during transportation if it is not a truss in operation. This makes the structure stiffer. In addition, on the dolly system, the steel module behaves like a simply supported beam, the support reactions have no big influence from the beam stiffness.

If your concern is for possible different response of the system (during braking or deceleration) with different natural frequency of the module (because of different stiffnesses), that is a good point. But the general design has not gone that far from my knowledge.