dock gangway

[wrightway]

i made a thread the other day and it seems it got deleted, anyways i went and looked at this ramp and the structure was different than what the guy told me, I know how to calculate the strength of the base frame but im not sure how much the upper portion adds to the structure, so here is the dimensions and the numbers i got

the ramp is
24" wide
30' long
30" tall railing with vertical supports every 5ft
2 frame railes are c5x6.7:7.48 in4
the railing and handrail are 1.5" .120 dom
and on each side there are 2 5/8 all threads that go to the center
the load i calculated is .04kip/ft which should give me a = load across the ramp of 2500lbs
i am not sure what the upper frame adds for support tho

 

[oldestguy]

Where are the lift points when in use? At each end? That cable in the picture is only for holding it when not in use??

Sure looks like a risky Rube Goldberg thing to me.

 

[wrightway]

yes the lift points are at each ends, at one end it is hinged to the dock, at the other end there are wheels so that as the tide comes up the ramp rolls on the float. In the winter the ramp gets tied up and the float anchored elsewhere to keep from beating the float and ramp up in rough water.

Yes, this guy used non pressure treated lumber for the decking and also left his dock attached thru a big storm so the piles got pretty beat up, he is re doing his whole dock in aluminum.
If you look coming off the top of the piles there is a 6x6 and from the end of that you can see a chain hanging down, that's what is holding the ramp up currently and is super sketchy.
I am not sure what cable you are talking about ?
If you are talking about the pieces that look like they run along the length of the whole thing, those are 5/8 rod with turn buckles, they go from the top of each end to the bottom in the center, I am guessing to help with deflection.

 

[oldestguy]

As to what holds it up now, I couldn't see it as a chain in the photo.

My guess is that the rods from each end to the center act as a part of a tied truss somewhat with the railings acting as compression members.

My view of the situation is that this is a very questionable walkway for people. To avoid getting stung with some legal action, should it fail someday,I'd walk away from that one now.

 

[oldestguy]

Assuming that any arrangement with the owner is such that you are not sued for problems down the road, one might make some simplifying assumptions that you have two somewhat independent structural systems. One is a form of kingpost trust-thing providing a mid span support to a beam.

Assuming the kingpost thing has the railings beefed up against buckling, as with angles attached full length, or other method of increasing the resistance to them buckling, perhaps some improvement of use can be made. Some form of calculation then would show a load capacity.

Posting signs at each end limiting the loading to say two people maximum at a time, then an over-load test would be required to prove that capability.

One way might be to place a barrel at mid span or several. With hoses run to each, measured quantities of water would be pumped out to them. Each would be fitted with a drain hose, ending at the dock.. Of course no one is allowed on the gangway while any water is in the barrel.

By measuring the water out and that coming back a form of proof test could be run. Owner should be aware that the whole thing may collapse under this test. However, liability for using it now or even with the test passing may just do the trick for a new properly designed gangway.

 

[wrightway]

the story behind this is the owner is replacing his dock and ramp with all new aluminum, and has offered me this one as i am doing some other non structural metal work at his property, it would replace my old sketch wood one,
i drew up a way i was thinking of beefing it up using 1.5" angle before having it re powder coated, it would make it more like a open web beam or a crane boom,
I like the idea of testing it with barrels, i could elevate on blocks at my shop and i actually have a bunch of 30 gallon drums i could fill with water place along the length.

 

[BUGGAR]

I suggest that the top rail does nothing structurally (kl/r and all that), unless you move an end support inward and load the resulting cantilever to keep the top rail in tension.

I responded to your original post then it disappeared. This seems to happen to posts I respond to. Sorry.

 

[oldestguy]

Referring to the last drawing, it would appear to mew that this is an entirely different thing. Re-building it as drawn and already setting the member sizes of course can be done, but that is backwards. Usually don't you assign the loads and size the members accordingly? However, building and then testing, I suppose will work, but why not do it the conventional way? 1.5" angles to me seems like it will not be able to carry much load.

 

[wrightway]

Well i decided not to get the gangway, the more i looked at it the more money it looked like i was going to be throwing at something that wasnt worth it, but there is good news, i decided to have a new one

what do you guys think of this,2"x.120sq tubing lower and upper rails and the diagnals are every 31" and are 1.5"x.120 DOM

 

[oldestguy]

Nice diagram you have, but to answer your question, I'd say: Get out the calculator, figure the section modulus and other data needed for designing compression members and those in tension by applying a given load. Then use the ratio of the maximum stress in the most stressed member to allowable to then calculate the maximum load it can take. You won't get that figure here.

 

[BUGGAR]

Top rail lateral restraint?

 

[wrightway]

Thank you for the compliment, yeah i actually got in contact with a company that builds gangways and managed to get a set of plans

 

[BUGGAR]

So you're going to use their plans?