Wind pressures on structure on top of boat 1

[jeffhed]

I have a client who wants to replace the roofing material on a very large tour boat. The structure is a 30'-0" x 30'-0" pole building with corrugated steel roof decking. The decking has aged and needs to be replaced but they need engineering to show that the new roof decking and the attachment will be sufficient to resist wind loads. If this is a structure on land that is no problem to determine the design wind loads, etc. However, with this boat being on water, I am not sure where to go to determine what the wind loads should be. The tour boat stays in the bay, it doesn't go out on the open ocean, so for preliminary reasons I looked at what the wind loads would be for a structure and used exposure D to generate some uplift pressures. However, this could be unconservative as this is only checking for actual wind speed not including the speed of the boat, etc. Does anyone know what code or document needs to be used to determine the required design wind pressures? I have done some internet searches and I get hits for floating boat houses, etc., but these are all structures that stay in one place and are not on top of a boat that is moving across the water, potentially into the wind.

 

[BridgeSmith]

I suspect if you combined the top speed of the boat with the highest ground wind speed that the boat could operate in, it would still be less than the ground wind speed required for design of it while it's stationary in port.

It's somewhat similar to what we do for bridges. We typically design for a 100mph wind (117mph gust), but we only use a 55mph wind speed in combination with traffic on the bridge, under the fairly solid assumption that there's unlikely to be trucks traveling when the wind is that high.

 

[TehMightyEngineer]

Probably some boat design guide would have the answer.

If this were my project I'd just take the load you have and bump it up by a reasonable percentage for estimate relative speed and off-shore wind effects. Remember that at some point your boat gets destroyed by wind speeds and they're not going to be too mad if the roof fell off after it broke its moorings and ran aground. How much cost are you actually adding to their project by being conservative on the loads? 5%?

I'd surmise that topography effects of whatever cove they park in during a hurricane are going to change the wind loads far more than relative velocity of the boat.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[jeffhed]

HotRod10, the highest ground speed you are talking about is a max wind speed that the boat could operate in. Meaning if wind speeds on the water were higher than that wind speed then they boat should not be taken out on the water, right?
This is what I have done so far, the boat is a dinner cruise ship that stays in Chesapeake Bay. The client tells me that the boat top speed is 15-20 mph max, so am treating this like any restaurant as a category II structure. Max wind speed for a structure in the area is 120 mph according the ATC website. This gives me a max wind speed of 135 - 140 mph. Which this feels reasonable for me especially considering the type of canvas boat coverings you see on smaller speedboats.

 

[BridgeSmith]

I think you've got what I was saying, but if you want to use 140mph, that would seem to be more than adequate. If it was me, I would make an engineering judgement call that the boat traveling 20mph into a 120mph headwind is an impossible scenario. Furthermore, the chances they are going to take the boat anywhere when the wind is blowing over 100mph are nil (just ain't going to happen), so I would just design for the 120mph wind load.

 

[TehMightyEngineer]

I'd say your loads are conservative enough; though not for the right reasons. Nobody is going to be driving a dinner cruise ship at full-tilt in a hurricane and be mad when the roof falls off (just blame it on the contractor using cardboard derivatives).

More likely is a "failure" will be encountered by the roof being considered too flexible or appear "about to fly off" during normal operations. The owner will need to feel like it's perfectly acceptable to drive at 20 knots over large waves during a 50 MPH wind; because he knows that his canvas roof was engineered by Jeffhed, PE and it looks "skookum as frig". I would ensure that you've satisfied flexibility and serviceability first; and then see if strength design for hurricane force winds controls.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[SAIL3]

no problem with the max wind speed sofar but that may not be the end of the story....but the old axiom in engineering "if you change the design, you own it"...based on that, other questions come to mind...
what affect does this new roof have on the stability/manueverability at various speeds and heel angles...
dynamic impact loads....
I would consider getting a naval architect involved to address these and any other hydrodynamic concerns...
it could be possible that this was never addressed even for the original design...

 

[jeffhed]

HotRod10,
Yes. We are on the same page. If the boat is sitting at the dock it would see the same design wind speed as any building, and I am sure as you have said and as TehMightyEngineer also pointed out that you would never see those kind of wind speeds on the water because you wouldn't take the boat out in 120 mph wind. Probably not even at wind speeds much less than 120 mph.

TehMightyEngineer,
Strength is not controlling the design of the roofing material. The roof will be built out of aluminum panels and the deflection is what is limiting the design.

Thank you both for your input.

 

[jeffhed]

SAIL3,
I am not sure if a naval architect was used previously, but the new roofing material will not change the profile of the existing structure, the old will be removed and replaced with the new roofing material. I am supposed to be receiving some photos of the existing roof to I can see how it was previously attached, etc. I am going to compare the existing connections with my new connections as another way to verify my wind loads with what has worked in the past. If my connections are less than the existing than adding a few more fasteners is not going to be a big issue.

 

[SAIL3]

the OP may be the first professional,in any category. that has been involved with this boat. I do not view it as strictly a structural engineering problem. Even if, the roof is replaced exactly as per old roof, the engr is implicitly approving the old design and all it's ramifications.
As for being exposed to severe storms, nobody with any sense would go out on the water under threatening conditions...the majority of mishaps on the water occur when unexpected/sudden storms arise and catch those out on the water unprepared.....