Staddle Stones vs Post Bases

[MortarThePoint]

I'm building a Loggia across the back of the house and to this point assumed I was going to use a galvanised post base, and settled on PBH120 because it it higher than the cheaper PPA and PISBMAI alternatives. It feels like a good choice to have the bottom of the timber post >150mm above ground level to avoid rain splash in a similar way that having house DPC there does. I have cast 600mm x 600mm concrete pads on top of slightly larger pads that go all the way down to 1.5m. I have put the top of that pad 225mm below house DPC to allow:

• 150mm patio to DPC
• 50mm paving slab height [or 63mm] (could be thinner slabs on a deep (40mm) mortar bed
• 1:80 patio slope so based on 1600mm total patio width that yields 20mm.
• 5mm slack

 

If using the Galvanised post base, the plan is to bolt them directly to the concrete pad and leave a 300mm x 300mm hole in the paving that I then fill with pebbles. I am a little worried that the post base may rust in time though, so that made me think of sitting the post (or even a shorter galvanised post base) on a concrete block of some sort. I have now learnt this is called a Staddle Stone and the post normally sits directly on this by means of a piece of rebar that is cast into the concrete block then sits in a hole in the base of the post. This seems like a nice approach.

 

Discussions with @Gus Potter identified that I need to think carefully about lateral loads due to things like logs getting piled against the posts. I am not sure which of the R numbers in the table below of post base capacities reflects this direction of loading. How is this dealt with when using a staddle stone? It occurs to me that I could have a piece of rebar (e.g. T20 = 20mm dia.)  passing all the way through the staddle stone and into a hole drilled in the concrete base. I expect I need a bed of mortar under the staddle stone and perhaps to resin the rebar into the concrete pad (and/or) staddle stone. Is that how it's done? Will there be enough lateral load capacity in such an arrangement though? The post may well end up being 150mm x 150mm Oak and I guess the rebar would pass into in about 75mm or perhaps a bit more.

 

 

https://www.strongtie.co.uk/en-UK/products/heavy-duty-elevated-post-base-pbh

 

[MortarThePoint]

Looks like it's worth considering stainless a steel dowel:

https://jbstone.co.uk/masonry-supply/staddle-stones/

[SteamyTea]

This is almost quite timely as I have been pondering how to make a cheap shed that is sturdy.

Wish I had seen this at 4:30 AM, then I would have had something else to think about during my 11 hours of driving.

 

[saveasteading]

2 hours ago, MortarThePoint said:

rebar (e.g. T20 = 20mm dia.)  passing all the way through the staddle stone and into a hole drilled in the concrete base. I

This. Because there is wind uplift to think about.

1m2 pad is rather a lot  but done now. The wind wouldn't shift that fairly obviously, or the plinth on its own.

Then there is your staddle stone which is a new term to me. That's for show so I'd put the stud / studs through it and into the plinth by about 100mm.

You can add nuts wherever you need level control.

 

The drawings look detailed so perhaps they specify bolts.

 

[MortarThePoint]

8 hours ago, saveasteading said:

This. Because there is wind uplift to think about.

1m2 pad is rather a lot  but done now. The wind wouldn't shift that fairly obviously, or the plinth on its own.

Then there is your staddle stone which is a new term to me. That's for show so I'd put the stud / studs through it and into the plinth by about 100mm.

You can add nuts wherever you need level control.

 

The drawings look detailed so perhaps they specify bolts.

 

The post wouldn't be gripping the rebar so should there be some form of retention there?

 

Something like the bracket below. It's rebar would ideally be longer if wanting to go all the way into the main pad.

https://www.amazon.co.uk/support-bracket-adjustable-Terminal-90X130X8/dp/B01LZGXEPV/

Edited September 11, 2023 by MortarThePoint

[saveasteading]

That sort of thing, yes.

I'm not sure that your staddle stone works though. It would need to be big and flat and sit very solidly indeed.

 

[MortarThePoint]

1 hour ago, saveasteading said:

That sort of thing, yes.

I'm not sure that your staddle stone works though. It would need to be big and flat and sit very solidly indeed.

 

Why would it have to be big and flat. If anchored the staddle stone to the concrete pad and the post is anchored to the staddle stone  I'd hope it can be something like 200mm x 200mm x 200mm.

 

It would be better if I could find a rebar post anchor with a longer piece of rebar as then it could pass all the way through the staddle stone and be structurally connected to the concrete pad.

 

I'd prefer something that doesn't end up visible, but I could bolt one of these to the top of the Saddle stone:

 

https://tradefixdirect.com/post-supports/simpson-concealed-post-base-1-1

[saveasteading]

That looks like a concrete plinth, which is standard and fine. I'm worried tgat your staddle idea will act like a rocker.

Pinned foundations are a standard thing too, allowing the column to rotate.

 

As calculations don't appear to be likely, you should use prprietory fxings, as you seem to be, and overdo rather than underdo.

 

When drilling and gluing rods, remember thst the first 25mm is considered as having no grip. 

[MortarThePoint]

Thinking about the resistance to lateral forces (e.g. heaped logs), horizontal movement of the post would require one of the following:

1. the staddle stone rocking over: I think this would come down to a battle of moments between the horizontal force applied at the top of the staddle stone and the vertical force applied though its centre (assume sold footing). The moments being about the X. Based on a 230mm high staddle stone which is 150mm wide, the horizontal force would only need to be (150 / 2) / 230 = 33% of the vertical force to start the staddle stone rotating if there was no tension resistance at the base of the staddle stone. Each post should carry about 3.6kN vertical loading of tiles alone, perhaps less for the corner posts (though hip tiles are heavy) so say 2.4kN. That means a horizontal force of 33%*2.4kN = 0.8kN or 80kg would start rotation. I'd hope that's a pretty high loading for things that might rest against the post? Some added 'safety factor' would come from using resin mortar to 'stick' the staddle stone down as well as securing a pin (e.g. 16mm x 120mm) between the concrete footing and the staddle stone. Also, some staddle stones are tapered adding to their width at base (180mm vs 150mm) and so improving the moments calculation.
   
    
2. the staddle stone sliding: If there is no pin or mortar/resin at the bottom of the staddle stone, the only resistance would come from friction. [link] says "PCI's Design Handbook says the concrete-to-concrete friction factor for dry conditions is 0.80. The latest edition of PCA's Concrete Masonry Handbook, Appendix A, gives a precast concrete-to-concrete masonry friction coefficient of 0.4 based on a safety factor of two." Again, using the vertical loading on the staddle stone as 2.4kN, that would need a horizontal force of 0.4*2.4kN = 0.96kN = 96kg for the staddle stone to start sliding. That has a safety factor built into the calculation, but resin and a pin coupling the staddle stone to the concrete footing would help.
3. the post getting pushed off the top of the staddle stone: There is a pin 16mm x 30mm to resist this. Assuming the pin is vertical, the post wouldn't get an upwards force to push it off the pin. Consequently, this could only happen if the pin was pushed through the wood of the post. I've no idea how much force that would take, but it feels like it would be a lot of force even for a softer wood like C24 let alone something like oak.
4. uplift lifting the post off the staddle stone: given the size of the roof and weight of the tiles this didn't seem to be a concern. Also, ceiling ties will help resist this movement.
5. something would have to break

 

 

It looks (table below) like this resin mortar used with just an M10 threaded rod embedded 60mm would provide a maximum tensile load of 4.5kN and maximum shear load of 8.6kN both of which are much greater than the resistance associated with the post's vertical loading and so greatly increasing the amount of load that would need to be applied to the post.

 

@Gus Potter am I thinking about this in the right way?

 

The picture of the failed staddle stone above scares me a bit more than this now I think.

 

https://www.fischer.co.uk/en-gb/products/chemical-fixings/injection-mortar/injection-mortar-fis-vl/539461-fis-vl-300-t

Edited May 8 by MortarThePoint

[MortarThePoint]

I've found some staddle stones without pins in, just pre-drilled 16mm (50mm deep) holes. That allows the use of stainless pins. Their straight sided not tapered.

 

There's a product called a Dowel Screw or Hanger Bolt that has a wood thread on one end and a machine thread on the other end. That should work well to screw into the end of the post and then resin anchor into the staddle stone hole. Some dowel screws have flats and some have a hex socket at the end of the machine thread. I think I'd want about 40mm sticking out of the timber unless I deepened the hole in the staddle stone. That's less anchorage depth than in the Fischer table, but should still be good. Also the dowel screws seem to top out at M12, so the 16mm hole is a little large, but hopefully OK. Another option is to drill a deeper 14mm hole on the other end of the staddle stone and use it the other way up. An advantage of an oversize hole is it allows a bit of centring tolerance.

 

https://www.westfieldfasteners.co.uk/Bolts-Screws-Metric/Dowel-Screw-Hanger-Bolt-Centre-Hex-Drive-M12x120-A2-Stainless.html

[MortarThePoint]

Some people just CT-1 staddle stones down and CT-1 does have good strength (CT1: Tensile: 2.9N/mm2=29kg/cm2, PGB: Tensile: 22kg/cm2, Shear: 11kg/cm2). 150mm x 150mm should have at least 24kN strength which is plenty, though that would assume perfect conditions. 10% of that figure would still be plenty. I think I will still use a threaded anchor though as that allows accurate alignment.

 

https://www.ct1.com/wp-content/uploads/bsk-pdf-manager/2020/09/CT1_Product_Info_sheet_TRIBRID_10_09_2020.pdf

https://www.ct1.com/wp-content/uploads/bsk-pdf-manager/2020/06/Power-GnB-Product-Info-sheet-12-05-2020.pdf

 

As far as I can tell PGB is a gloopier CT1 with better initial stick (i.e. grab).