Anchor Pins to Ledge

[rmallaire]

Hello all,

First off, please excuse any incorrect terminology, as I am a novice to this subject.   I am unfamiliar with fasteners that would anchor to ledge. It is fastening the base of the posts to the ledge that I need to become familiar with.  I will need all hardware. Any help would be greatly appreciated. I have began researching different anchors for concrete but didn't know if there is something specific for anchoring to ledge. Ideally what I am looking to install is an anchor pin with a fastening system that will allow a base plate to be mounted on top, then a 6X6 or 8X8 post will mount on top of it.

I am building a small structure (20 X 20 cabin) on a piece of land I own in northern New Hampshire. The area is mostly ledge, strips of granite, field stone and shale.  Ideally, I would like to install pressure treated posts for the carrier beam to be placed on (8' on center- 9 total). The floor joists will cantilever over the carrier beams by 2' all around.  I would like to keep the design as simple as possible, while adhering to fundamental principals for the correct methods of construction.

Thank you,

Randy

[flyingvan]

  I'm guessing by 'ledge', from your second paragraph, you're talking about exposed bedrock?   A picture would help.
   If that's what you mean, buy a hammer drill.   Drill 5/8" holes at slight angles and epoxy rebar in.  Use the rebar and 'tie in' horizontal rebar, build a form around it, then fill fith concrete---set your metal post holder right in the concrete.
   Make absolutely certain the stone is structurally stable and capable of holding the load.  If you can't hire a local geologist look around and see what other people have done, how old it is, and how well it's held..
   

[Redoverfarm]

Randy I am not sure from your description exactly what you are trying to do.  If you are trying to attach brackets to the bedrock or "ledge" as you describe the only way is to drill into the bedrock and use wedge bolts or epoxy bolts.  Here is what I am describing as wedge bolts.  They can be obtained in various diameters and length.  I would not go with anything less than 1/2-5/8" and would probably prefer 3/4".  A drawback would be the amount needed to hold the base on a plate of such little demensions.  Here is what they recommend as far as spacing of the bolts.

The forces on a Wedge Anchor are transferred to the base material in which it is installed. If the anchors are installed too close together, it can cause an interaction of the forces, thus reducing the holding power of the anchor. As a rule of thumb, the expansion anchor industry has established a minimum standard of ten (10) anchor diameters for spacing between anchors and five (5) anchor diameters from any unsupported edge. When vibration or sudden impact are part of the load conditions, spacing between anchors should be increased - See more at: http://www.concretefasteners.com/anchors-fasteners/thunderstud-wedge/technical-specifications.aspx#sthash.FvC3b31S.dpuf

http://www.concretefasteners.com/anchors-fasteners/thunderstud-wedge/?gclid=CITtisCVy7kCFYhAMgodUz4AKA

Now if you are just setting pins to use to tie rebar in a footing you can accomplish this by drilling two holes at an angle so that 1/2 of the length of the rebar is in the rock and the other half is sticking out but crossing each other above.  Once the rebar connection is made and the concrete is poured around them the "X" design will will prevent upward lift or movement.  I used this in a large concrete pour on a bridge abutment to add to the heigth of my bridge and wingwalls.  Of course I used larger sized rebar ( #10-12).  It was considered a cold joint which had already been poured previously.  I had thought about using epoxy & pins but with the amount needed 56' in length of both ends it would have cost a fortune for the epoxy and the aforementioned method was used at a fraction of the cost and has stood the test of time.

The major problem with using wedge or pins is the rock consistancy.  Limestone, granite or similar hard rock is pretty much compatable where shale is not.  Another problem with setting the fastners ( Simpson or similar) for your pressure treated lumber is that the base area needs to be fairly level or your post will not be plumb causing the above structure to be out of plumb as well. 

[Don_P]

I would like to keep the design as simple as possible, while adhering to fundamental principals for the correct methods of construction.

There's the rub, although Redover gave good advice on the attachment of plate to competent rock, the use of a 6x6 pier in a simpson bracket that is not intended for lateral load is where the problem comes in. Those bolts won't fail if the post tips over first. For something like this poured piers no taller than they are in each width direction rebarred and grouted into the shield rock would probably be a geotech's solution, and that is really a job for one.

[pocono_couple]

no advice on the  problem, but i used to live in the lakes region..  sure is plenty of ledge to go around in NH !     my next build project is going to be in Maine..   the ledge there is 4 feet down..   i don't think that we will get a full basement, but that is ok.. how far north are you in NH?  jt

[rmallaire]

Flyingvan, 

Thank you for the tips. Yes, the ledge I was referring to is exposed bedrock. I will work on getting pictures uploaded this week. From what I have taken notes of, the bedrock is a mixture of granite, garnet, quartz, dolomite, shale, schist.  I leveled some building sites with an excavator- where the bedrock was found, the excavator could not continue. The bedrock runs as long deep strips with (not boulders). 

Drilling into it with the hammer drill and 5/8" bit is exactly my plan. Does the epoxy work for areas with extreme temps? I am in a climate zone 3 in northwestern NH.

[rmallaire]

Don_P,

Thanks for the reply. I do intend on drilling two holes into the rock and setting the "X" pattern rebar for each sonotube.  Just to clarify, the length of the sonotube should no exceed its diameter?