Beam helper on stair stringer

[MortarThePoint]

On 04/11/2023 at 21:50, Gus Potter said:

The weak spot is at the top as you are fixing into the end grain where the notches for the treads can split off.

 

Follow the load path which is.. each tread is on a non continuous piece of timber to some extent... the notched part. The notched part transfers the load to the main continuous part of the stringer. At the top you have three fixings into the end grain, probably only two of these are really doing the work, hard to tell. Assume that at some point the timber will split to some extent.

 

If you plate the outside stringer with ply and glue any potential problem should be mitigated.

 

Sorry, I should have quoted more of your original post. The nail plate would be to mitigate the chance of the stringer splitting at the screws at the top.

 

I think that the screws at the top don't really perform much of a load bearing job as the stair would still stand without them since the steel angle is rigidly bolted to the steel beam so couldn't rotate out of the way you allow the stringer to fall.

 

Here is a mock up that shows where I was thinking I could glue the nail plate. The three screws per stringer are shown as arrows. The nail plate would just be in an attempt to stop the timber from splitting. Outermost stringer removed for visibility.

 

[Gus Potter]

Ah now I see! Have to say.. you're quick with the representations and thinking on your feet.. do you want a job?

 

The more screws / nails you put into the head of the stringer the more likely it is to split... leave it alone. We have all sorts of rules for timber fixings, edge and end distances, the load direction and how the load is orientated to the direction of the grain. What you propose is messy.

 

Can you..

 

Using a structural glue and screws (provides clamping for the glue and a bit of redindancy in case the glue is duff) to thicken the stringers on the inside with MARINE PLY not OSB. The grain of the outer ply should run horizontally.

 

Say stringer is 33mm thick + 18 marine ply = 51mm. Make the ply cover a good area of the stringer, may past the next tread down.

 

Bolt a 70 x 50 x 6.0 thk steel angle to the vertical leg of the angle that sits inside the bottom flange of the UB. Suggest 2 no M10 8.8 bolts though the short leg into the angle section you show. The 75mm leg runs parallel to the stringer.

 

Drill the 75mm leg of the angle and fix that through the ply and into the stringer using short M8 x 40 long coach hex head screws. My first guess is that you need 4 coach screws per angle as the stringer length is pretty short I seem to remember from your previous posts. The long leg keeps the fixings away from the end of the stringer. The ply prevents spliting of the stringer end.

 

Below is a screenshot of the edge and end distances you need to comply with based on BS 5268 part 2 2002. Note the load direction.. you have clocked that earlier when you recognised that some of the fixings may not be doing much. If in doubt always assume the worst load direction. d = the diameter of the bolt.

 

Some bits from the table are key.. edge distance.. the top coach bolt needs an edge distance of 4d so it doesn't split the top of the stringer off. The bottom fixing is pushing into the grain (meat of the stringer) so only needs 1.5d.

 

You want to keep the coach screws 7d from the end of the stringer (as the grain is at an angle to the load direction) to be conservative. 7 * 8.0 mm dia = 56mm. For steel with a drilled hole the edge distance is 1.4d = 1.4 * 8 = 11.2 mm... say 12.0mm

 

12mm + 56 = 68 < the 75mm leg of the angle.. and there you have a compliant connection even once you take into account fabrication tolerances. If push comes to shove we can use more detailed calcs to refine the connection design.. keep it simple and avoid.

 

I mentioned 4 screws. They need to be 4d apart vertically = 4*8 = 32 mm. If you draw that out you may have room for more fixings in the vertical plane if need be so.

 

For all. The above table is a handy reference if your learning about TF construction.

 

 

 

 

 

 

 

[MortarThePoint]

11 minutes ago, Gus Potter said:

The more screws / nails you put into the head of the stringer the more likely it is to split...

This is why I was thinking it would be a good idea to glue (CT1) the nail plate to the stringer rather than using any nails. The glue would push through the holes in the nail plate making for a very string bond

 

14 minutes ago, Gus Potter said:

Bolt a 70 x 50 x 6.0 thk steel angle to the vertical leg of the angle that sits inside the bottom flange of the UB. Suggest 2 no M10 8.8 bolts though the short leg into the angle section you show. The 75mm leg runs parallel to the stringer.

That's a nice arrangement as it avoids any screws into end grain. Given I already have the screws into  the end grain, I am less inclined to drill / screw any more holes into the stringer. The rotation caused by any loading on the stair (or the stairs own wait) is to push the stringer into the angle that sits inside the bottom flange of the UB so it feels I figured end grain attachment would be OK. Our American cousins do this a lot and one of their common approaches is to place a piece of Plywood onto the face of the the trimmer joist and then screw through that into the end grain of the stringer.

 

I have four 45mm wide stringers so hopefully there is a fair degree of redundancy at play.

[Gus Potter]

If your half landing at the bottom is buttressing the upper set of treads / stringers against horizontal movement then you actually can show you need minimal fixings at the top.

 

Think about a ladder against a wall with someone holding the bottom still. The load at the top of the stringer is horizontal only (as the top of the ladder can slip down the wall) which in your case is pushing against something solid.

 

If you are confident your half landing can resist the horizontal loads then your problem is solved! All you need is a few fixings at the top of the stringer to stop the stair moving left or right in the plane of the top landing. Does this work?

 

 

 

 

[MortarThePoint]

50 minutes ago, Gus Potter said:

If you are confident your half landing can resist the horizontal loads then your problem is solved!

Yes, the half landing frame is very solid and has lateral timbers bracing against the wall opposite 

 

I was mainly thinking about the splitting risk you identified, but that wouldn't be due to loads

Edited February 4 by MortarThePoint