deck structure analysis

I do this kind of analysis all of the time with my work. lets see if I can make this make sense in a few sentences.

look at each element as a "free body". The loading is actually a couple on the deck surface, the structure of the deck assembly ahead of the winch will have it bending in one direction, and the structure behind the winch mount will be in the opposite direction, so you have to design for bending in both directions (typical couple loading condition). If the mounting plate is stiff enough than you can assume it spreads the total load evenly out over the supporting stringers than that makes your job that much easier. you can usually assume the part of the deck beyond the side edges of the mounting plate are not contributing to supporting the deck loads, this is conservative since it will actually contribute to supporting it.

If you for example have 160,000 ft-lbs of bending moment generated by the fully loaded winch (max cable tension times the height the cable will be above the deck), and you have say eight stiffeners under it 160/8= 20,000 ft lbs each. so you can solve for bending in each individual "T" beam, or I beam bending moment. You have to calculate the bending moment at different points along the deck/stiffener combination to determine how far out the heavy stiffeners have to be run. when you get far enough away from the winch so the bending moment the normal deck/stringer combo can safely carry that is where you end the heavier stiffeners. You also have to add the tension load to the stringer loading, so you will have a combination of bending and axial loading along each T-beam or I beam you are examining. You have to calculate the shear along each stiffener to determine the size and spacing of the fasteners (or size of welds if it is a welded deck). Typically you can ignore any other variable or live loads when designing for maximum winch loads, or there might be load combinations you need to consider (misanalysed by super position).

You also have to consider off angle pulls on the winch, unless you have a large cable guide or pad eye to keep the direction of pull in the same direction. the safety factor should accommodate small out of plane angles.

Typically you take the worst case loading condition, than add a safety factor determined by the class of boat, and the likely hood of a dynamic loading (such as using the winch in a towing configuration where there could be some impact loads). Typically the factor of safety would be about 1.5, but could be much higher depending on the intended use of the winch. Again typically the class rules of the ship usually call this out.

I hope this makes sense. Good luck.

 

Petros: ur post is very helpful man. i just need to get clear on some stuff.

if i'm having both longitudinal and transverse stiffener ,as in my case, under the mounting plate then how should i divide the load by them?

here i'm confused abit cause u said above to just divide load by stiffeners so why do u mention a combination of deck stiffener here. hope u can clarify

 

if you have a grid of stiffeners in both directions you could have an indeterminate structure where the loads is going in multiple directions. Usually that is not the case, the designer will choose to carry the load in one primary direction, and anything that is going the other way is just there to prevent buckling or twisting of the main load carrying members (tall slender I beams will want to roll over if not laterally braces). Usually the detailing on how the members intersect will give you a hint as to how the original designer was thinking on transferring the loads. IOW, the detailing will usually indicated the assumed load path through each connection or intersection. It is also possible that the loads were intended to be carried in one direction to a header member cross wise to the others, and this header member will distribute the loads to the heavier girders on either side of the winch location. I looked again at your plans and am a bit baffled, at first I thought the "new beams" were there to spread the loads out to the stringers more evenly over the width of the mounting plate since the stringers are actually in the direction of pull on the cable (as I understand your illustrations). But I also see these short transverse "new beams" are also very deep and attached to a bulk head at one side, but not on the other. these deep "new beams" appear to end attached to no particularly heavy or strong supporting member out in the field next to the winch mount. I see something labeled "bkt" for what? bracket? that should be an extra heavy stringer. Not knowing a lot about the details of this install it appears to be a very poor design, when the deck flexes where it is not reinforced it will load up the stiffer side almost certainly overload it. The deck has to be stiffened so it distribute the loading evenly over the members. IT will introduce complex transient loads into the deck plating where these cross beams end. If there was a deep heavier member here parallel to the deck stringers that spread the load lengthwise along the deck I can understand this installation, but not what it appears to be now.

when you look at the distributed load on each stiffener, you treat that individual stiffener in isolation. Than looking at that stiffener only, the bending moment will be highest where the mount ends, or at the location of the cross beam, and as you get further from this location the bending moment gets smaller. the idea here is to determine what the maximum moment the T-beam or I beam stiffened deck will carry without extra reinforcement, and you calculate how far away from the mount that bending moment will be. That is where the extra reinforcement ends, where you have spread the load out far enough so the nominal strength of the deck will safely carry it. the higher the bending moment you introduce into the deck, the larger the extra reinforced area has to be to spread it out over a larger area.

but that installation appears to put the loads into a bulk head on one side, but has the deck carry the bending moments along the stringers at the other, which is an unsymetric way to spread out the load, not typical design practice. I would not assume who ever laid that out knew what they were doing, or they are designing enough over kill into it that the transient asymetric loading is is small enough to ignor. this is wasteful and add unecessary weight and costs, but it is possible that is what the original designer had in mind. When in doubt, make it really stiff and strong. That is not really "engineering" but using brute force overkill to carry the loads.

 

then i will divide the load by the longitudinal stiffeners.for future designs u suggest designing with longitudinals and then solve buckling issues with transverse beams. this design is not mine it was just handed over to me.

why will that be? i thought the max moment location is affected by where the moment is applied across the beam, we are talking here about the reaction moment not the applied moment, right?

here is the thing confusing me, when u say stiffened beam deck does that mean u take some effective length of the deck plate and add it to the beam cross section before making the clculations or is it just a phrase u use to stand for individual beams?

the second thing is lets say the extra reinforcement is no longer needed midway to another transverse stiffener do i cut it there or just stretch it to attach it to the transverse stiffener.

 

perhaps a diagram would help. It is back to school: Study the attached sketch I made for you. the deck has a certain bending capacity, and the winch imposes a bending load larger than that capacity over a portion of the deck surface, where that imposed bending load is in excess of the deck capacity, that is where you must reinforce the deck to the capacity of the imposed loads (plus the safety factor). just to clarify, the applied moment is that which the winch puts on the deck, the reaction is the load in the deck members.

Study this diagram and see if it makes seance. when I said "cross beam" I was referring to the structure on your drawing that says "new beams", these would serve to spread the load from the winch base more evenly along the stringers. since it transmits the winch loads to the deck this is where the moment would be highest (see moment diagram).

It would be useful and good practice, to end the doubler or reinforcement at a new light cross beam or intercostal (or carlin), or on an existing one (so yes you can can extend it to the next cross beam if that is easier and cheaper than adding a new cross beam) just to give it something to attach to and not create a concentrated load at the doubler ends. but it should not carry any significant loads since the total stress should be below the capacity of the original deck and stringer capacity anyway.

the deck stringer plus the deck skin will act like a "T" beam and you can use the deck plus stringer to distributed the loads IF the deck is a continuous load path and will transmit the applied loads to the stringer.

 

i guess u are talking about an effective length of the plate here so how should i take this effective length?

ur diagram cleared what u said but one thing, looks like u are taking moments as zero at the edges so is that cause beams are too long, or are u taking the edges as simple supports?