bending stress in longitudinally stiffened plates

[greycloud]

Hi there everyone

I'm working on calculating the stresses in a winch foundation installed on the main deck of a barge. the pull on the winch causes both axial and bending stresses in the foundation under the winch so here is the problem. I looked hard for an equation that deals with critical buckling strength due to bending and one for getting the applied bending moment on the foundation cross section but couldnt find any. the foundation is composed of a plate with transverse and longitudinal stiffeners attached to it beneath.

I hope some one can reference me to such equation or to a book talking about this matter.

Thanks and help appreciated.

 

[desertfox]

Hi

Post a pdf drawing of the situation please

 

[greycloud]

sorry I didnt reply earlier but I was busy

I have attached a drawing of my problem. thanks for your interest

 

[desertfox]

hi greycloud

All I could find was this:-

http://www.ce.jhu.edu/cfs/cfslibrary/SSRC Guide 2009 Ch 4 plates Schafer version.pdf

 

[greycloud]

Thanks alot desertfox but I went through this content before and it doesn't include a case similar to mine. what do engineers usually do when faced with such specialized cases. I'm new as a design engineer so do you have any tips?

 

[desertfox]

hi greycloud

I would do an analysis on the rib in isolation, so lets say the force is divided by say two ribs and then do an analysis on a single rib carrying an axial force (strut), followed by an analysis of a simple support beam with a constant bending moment applied across the whole beam length.
This should be very conservative because the rib cannot act like a simple beam because its welded to the baseplate, you could actually look at the cross section of the plate and rib and workout the "I" value for the composite section and then analyse as a beam and strut once more.
That's all I can think of unless you have access to FE but I'm guessing you haven't.
Incidentally how is the plate fastened to the deck?
desertfox

 

[SAIL3]

The first major problem is to accurately define the loading conditions.
Is the item being winched on the barge itself, if not, then a host of questions come to mind.
What are the possible directions of the winch load.
What impact factor to use.
Is this an existing barge.
Not enough info supplied to give any meaningful solution.

 

[BAretired]

I agree there is not enough information to provide a meaningful solution. It would be a very difficult problem to execute by hand.

.1 Can the winch be considered a rigid body so that the plate rotates uniformly over the area covered by the winch? If so, that would simplify the problem slightly.
.2 Are the ribs normal to the plate or at an angle as shown?
.3 What are the dimensions of the plate?
.4 What are the dimensions and spacing of ribs?
.5 What floor loads, other than winch loads can occur simultaneously?

BA

 

[TLHS]

This is the sort of situation where you want to be robust, so I think a hand solution is fine because you should have a healthy factor of safety anyway. I would start by doing a preliminary sizing based on a point moment applied in the middle, spread across ribs depending on how everything's attached. Assuming couple forces might be a better idea, depending on how the winch is built, but I probably wouldn't start with that assumption.

It's probably not as hard as you think it is, because with a small span you hopefully aren't in a range where buckling is particularly significant.

Start off by figuring out your design load. This is important because you can easily make different sets of assumptions that result in loads several times larger than other sets of assumptions. It will depends on how the equipment is being used. It may be the pull capacity of the winch if things are controlled, or it could be something like the breaking strength of the cable or another component if there's a possibility of crazy things happening (if this is a tow cable you could have the object being towed catch on something at speed, for instance), alternatively you can design it as a breakaway item where you size the connections for a working load and then make sure everything past that is stronger than the connection by some factor so the winch will tear away if there's a problem. Talk to the owner or operator to figure this stuff out, because it really depends on operating conditions. If you end up using some sort of working load as your basis, you'll want to figure out an appropriate impact factor as well. Also have a good think about what the worst case pull angle is.

Okay, so you do your preliminary design based on moment. Leave a bit of extra meat in the design and then split out a sample single rib with the tributary area of plate to check if you're in an area where you have a buckling problem. Check the deflection of this section under the moment and determine whether p-delta is a concern (just do a quick multiplication of your deflection and load and see if it's in an order of magnitude where it's significant, if it's sort of significant, throw a reasonable multiplier on it and add it to your design moment as a conservative approximation of p-delta at this stage), if it's really significant you have too much deflection and probably want to stiffen it up. Also, check whether your deflection is reasonable from a visibility and servicability standpoint.

Now take your axial load and check the rib and tributary area as a column. Where are you in the slenderness range? I'm guessing you're pretty stubby and you have a resultingly high buckling strength. If you are, and you've taken some approximation for p-delta, I'd likely just sum my utilizations for moment and compression together and see where I'm roughly at.

You now have a preliminary design that's reasonably conservative depending on your approximations. Look at the sizes you've got and maybe run a few alternate assumptions to try and see how much a less conservative design might save you. If this sensitivity analysis shows that you're not going to save a lot, then run with the conservative design. If you're oversized by a lot then it might be time to break out the finite element software or to try and do a more complicated stress and buckling interaction analysis.

 

[JStephen]

So do naval architects and engineers have their own structural standards, or do they just wing it and pluck allowable stresses out of the air as required?

Consider putting a base plate under the lug/winch to distribute the load across additional stiffeners if required.

AISC has design information for stiffened elements of wide width, which could be applied to the deck plate. It looks like compression in the flange away from the deck would give a laterally unsupported flange which might be the controlling condition.