Concrete footings on sloping solid rock

[AV Elec]

I am looking to mount some posts onto rock which is sloping. I was thinking of chemsetting some steel rods into the rock, then forming it all up and filling with concrete.

The posts will be anywhere between 4 and 5 metres tall and supporting a deck. Does this look to be a suitable method? Since there are only going to be 3 done this way I am more than happy to over engineer it and was going to put reo and blue metal in the footing as well.

I really appreciate your thoughts.

[brynk]

if you are sure it is solid rock then drill some chemical anchors into it & set them. level your post onto the rock itself by using packers or two nuts on each thread, the bottom to hold the base-plate up at level then the top to tighten the base-plate to the bottom nut. finally, use an epoxy grout into the gap to protect the packers, nuts & threads from corrosion.

4x m18's would give you a massive amount of shear and bending load-bearing capability as long as the footings can accomodate it. the most expensive consideration with this method is the size of the holes that you drill - each step up is an additional cost on the drill-bit(s) you'll require, and you will need to go reasonably deep - at least 250 by gut feeling. a mitigating factor of the chemical anchor is how much contact area the epoxy resin has with the threaded rod and with the material it is being drilled into. the other is the rock type - if it is a sandstone or shale you will need to deeper than if it were an igneous or metamorphic rock - fortunately it will be easier to drill.

if your posts are going to be that high you'll need cross-bracing between the posts to reduce the bending load on the footings. what is the footprint of your deck? how does the land lie underneath it? i notice on a previous post somebody mentioned access to under the deck to clear out detritus and undergrowth to ensure no flames can take root under the deck itself. it seems by your heights you will have no problems with this - by most councils it is this height that will demand an engineer or equivalent certify the design for your approval, and the development application will require a licensed builder to undertake construction of the footings and sub-structure at the least.

you might at least engage an engineer to do the design for you, whether you apply for the da & have it built with approval, or not. then you'll know that it will be structurally sufficient. part of the engineer's fee will be inspecting the construction to ensure that it is done in accordance with their design. the engineer won't do the design until a geological survey is performed and a report produced for them to work off - this might cost anywhere from 300 up, depending on your location.

regardless of the material used, cantilever as much as is reasonably possible as this will cut down your material and footing requirements. there are hardwoods that are fire-resistant that may be cheaper than steel. but then, as soon as you are paying for a crane to lift members into place it will be cheaper to use larger sections that span further.

regards
brynk

[AV Elec]

Thanks Brynk, great advice.

Under 2/3 of the deck is an old concrete slab at 2500mm to deck. Under the other 1/3 is very steep rocky land approx 4500mm to deck.

The footprint of the deck is 4600X 6000mm. It will be positioned against the back of the house with a 6000mm bearer parellel to the house. I really want joists to span the 4600mm to the house so I can use the area underneath.

A DA is a must. I cannot get a crane in there, but could possibly use a genie lift for bearers, joists, etc.

I have gone over the options for a while, and I think the easiest way is to use onesteel to design it in steel, get an engineer to confirm the footings, etc, get a builder to assist in the footings and bearers, and I do the rest solo. At least that way I am not liable for the design and structure, and it should really be like putting together ikea furniture with distructions.

I have 2 trades and over 15 years experience, I have done heaps with steel and mounting to masonry, but only basic woodworking, and no concreting over the years.