Moment arm canteliver bolt question

[sam37]

Question for everyone, I have to bolt a steel channel to an existing pile. I have 3 ft of lateral earth pressure that will be pushing on the channel, and it will be cantilevered to the pile. To be able to get it to work, I will likely need to use a through bolt, then weld a knife plate to the channel. The question i have is the spacing down further down the pile, I have a few feet to work with and am wondering how far I should extend the channel and how that will effect the tension in the knife plate/shear in the bolt. Also, is the end just going to be in full compression, so all I need is just like a small lag screw? It wont be trying to pry away right?

Please let me know of any ideas or things I should consider in something like this.

 

[Agent666]

Draw your moment and shear diagrams, work out the reactions and put the connections where they work. Ultimately depends on applied load and capacity of the fixings which you haven't actually given.

Yes bottom will be in compression under the loading scenario you've noted.

 

[BAretired]

In the diagram below, W is the horizontal force due to lateral earth pressure. The shear force on the bolt is W(a+b+d)/d which tells you how the bolt force varies with dimension 'd'.

BA

 

[rb1957]

not than BA's wrong, but a more conservative approach would be to say that the compression 1/2 of the couple resisting the moment is a triangular dist'n between the steel chnnl and the 12" pile (instead of being a point force at the lag screws). this would change "d" in BA's bolt shear to "2/3d".
and "a" is 1ft.

another day in paradise, or is paradise one day closer ?

 

[BAretired]

With the expected roughness of the pile, the assumption of triangular distribution is questionable. It might however, be a good idea to provide a washer or small bearing plate at the screw to ensure the channel is clear of the pile except at the screw.

BA

 

[KootK]

It might however, be a good idea to provide a washer or small bearing plate at the screw to ensure the channel is clear of the pile except at the screw.

I was going to suggest something similar. A channel bent in weak axis isn't going to be super stiff and there might be potential here to develop prying action whereby the center of compression winds up being closer to the top than a rigid analysis would suggest.

Additionally, how do we feel about the detailing of the bolt at the top? While it's a legit wood connection, I worry about it's durability in a permanent load / earth retention situation. I'd hate to see a chunk of wood rip off of the channel side of the pile at the bolts. If possible, I'd prefer a connection that delivered the top shear to the dirt side of the piles somehow.

 

[BAretired]

Is the pile wood or concrete? In either case, I agree with KootK that the upper connection needs further thought.

BA

 

[KootK]

Is the pile wood or concrete?

It would seem that I just assumed that based on the mention of lag screws...

 

[rb1957]

wouldn't you rather carry the tension load as shear on the bolt (rather than tension) ?

the pile can fail similarly with the tension bolt (as with the shear bolt proposed).

another day in paradise, or is paradise one day closer ?

 

[KootK]

wouldn't you rather carry the tension load as shear on the bolt (rather than tension) ?

Nope. The bolt in tension is way more reliable than wood in cross grain tension (as shown in OP sketch).