How far out can I extend a structure supported on one end?

[bdog]

I have a small pond at my place. Roughly 100'x200' . I want to build a fishing dock but I don't want it floating on the water I want it freestanding. I plan on digging a massive hole with the backhoe beside the pond and filling it with concrete to anchor it to and would like to extend a structure out over the water maybe 5' above the high water level. Could fish and dive from it. I was thinking two or three large I beams maybe 15' total width with smaller I beams connecting them and wood planks on top. Due to the 5' height over the water I will be able to get a bit of an angled brace underneath it. I have attached a crude sketch.

How far can I extend it out and what size beams will I need? I never envision more than 10 people on the dock. If I could get out 30' it would be great but I don't know if that is possible.

The other option is to bring in a crane and build something that spans the entire 100' width which would likely cost a whole lot more.

 

[Rock knocker]

What are we? Ask an engineer? Ten people 30' out on a cantilever over a lake? Ever hear of the KC Hyatt?

 

[CalG]

I hear floatation is bio friendly.

 

[bdog]

Maybe 30' is a stretch. 10' maybe 15'?

I was planning on large I- beams for both the vertical an horizontal thinking at least 2'. I guess the alternative is making it float but I was wanting to avoid that.

I found some calculators online for various loads, cantilever length and beam sizes. What is an acceptable deflection? If the deflection for a single beam is 1/2" would two beams be 1/4"?

Ten people would be the absolute max and probably never happen. Most likely it would be 4-5 with half of them kids under 100lbs.

 

[dragoneggs]

This is simple cantilever beam analysis. You will definitely be sizing for deflection rather than strength. Yes, if you double the number of same sized (cross section) beams you will halve the deflection. Personally I would want the flex to be unnoticeable. You also probably need to pour a significant concrete footing since you will have a large 'overturning moment' to react at the shoreline and I am assuming your soil will not be rock hard that you are burying your verticals in. The structural design of the beams and platform is easy. It is the mounting/grounding to earth that needs understanding. You don't want it rotating out of the hole at your reaction point. Also you are on the right track with gussets to shorten the cantilever. Lastly you need to consider 'live' loads not just 'dead' loads. Not just people standing but jumping up and down. I would double your max 'dead' load and then size beams based on deflection.

 

[CalG]

Calculation of the structural loading and support structure is the easy part on the internet.

Your soils and base on the other hand are completely unknown.

It's unfair to even ask the question unless the home work is done.

Have you dug any holes? hit any ledge? Found any bog soil that goes for ever?

Home work! Cantilever's are not for the ignorant.

 

[bcp]

I'd do it more like this.

Bruce

 

[bdog]

No rocks and no bogs. Just dirt/caliche down to at least 16'. The minimum concrete I can get delivered is four yards so I plan on using at least that much. It weighs what 3,500lb a yard so I will have a 14,000 lb of concrete minimum. I wouldn't mind digging a 2' wide trench 10' deep and 15' long parallel to the bank and sinking all two or three vertical beams in in and dumping a whole concrete truck in there. Would be easy to do with my backhoe.

 

[MossRoad]

What you need is a 60' bridge set 29' out over the water so you'd have 31' behind the main pier. It'll balance itself. Make sense? Then a counterweight on the far land side to account for the up to 10 people, with some fudge factor increase for safety.

There's another easy way to figure this out.... buy an engineer some dinner.

 

[MossRoad]

I'd do it more like this.

View attachment 510528

Bruce

HA! Like you said!!! I was typing while you were posting. :thumbsup: