How to fix a floating bench on a concrete sleeper wall?

[StephaneDubois]

Hi,

I'm getting a retaining wall put into my backyard to create a paved entertainment area. The area is approximately 4x12m and the retaining wall is 10m in length. Along the wall I would like to anchor a floating bench which will be covered in decking wood.

Here are my challenges: 

1. Drilling holes in concrete sleepers: Based on the research I have done I cannot find any definitive guidance on drilling holes into concrete sleepers. My intuition tells me that as a general rule it is probably not recommended to drill holes in concrete sleepers as that would weaken the sleepers... The other option I considered was to weld a metal frame to the steel post to attach the bench frame to, but I don't know and don't have the equipment to weld. Any welders in SA welcome to help !

I'm also interested to hear from people who have drilled holes in concrete sleepers and what their experience was!

So I'm back to the drilling holes option. The only concrete sleeper company I could find that talks about drilling holes in sleepers is OutbackSleepers (https://outbacksleepers.com.au/wp-content/uploads/2019/11/Outback-Sleepers-Product-Guide-Website-Ver...). They explained how to drill holes to set up steps between two retaining walls. The load on those sleepers would be less than the load of my 10m bench but at least it is a good start: They say in their product sheet that you could drill 12mm diameter holes at minimum 60mm depth! 

For my project, the holes would need to be ~450mm, maybe ~500mm apart for the bench frame to be at ~500mm centers for the decking. This means that I would have 4 sets of holes per 2m long sleepers which again worries me from the perspective of keeping the concrete sleeper wall structurally sound... 

2. Anchors: Under the assumption that I can drill 4 sets of holes in each 2m sleepers, I will need to choose an anchoring solution.  Following some research, I have opted for the chemical anchor option (as opposed to mechanical anchor) to avoid putting too much force on the sides of holes and to reduce the risk of cracking the concrete. 

3. Brackets:  I will need to use steel brackets to support the wood frame for the decked bench. I have been looking around the internet for steel brackets and the only ones I can find that seem suitable for my project are the Hoppo steel brackets (https://www.demak.com.au/shop/hardware/brackets/the-hoppo-black-steel-seat-bracket/).

If anyone has used these brackets or seen/used other brackets for their floating bench I would love to hear from their experience.  

4. Force (moment) on the retaining wall:  The final challenge that I'm trying to address is figuring out whether the wall will be able to sustain the load from the 10m bench when fully loaded with people. The retaining wall will be 1.2m high, engineered, and built by a qualified contractor. 

Looking forward to hearing from the community on my challenges but also anything else I have forgot to consider!

Stephan

[Noyade]

Morning Stephan @StephaneDubois 

How thick are your sleepers?

"The retaining wall will be 1.2m high, engineered, and built by a qualified contractor."

Have you discussed your floating bench idea with your qualified contractor? What does he or she think?

I may have the math wrong, but would the Demak brackets alone cost you $1,500 minus 15% discount?

[MitchellMc]

Welcome to the Bunnings Workshop community @StephaneDubois. It's wonderful to have you join us, and many thanks for your question about installing floating seating on a retaining wall.

The best person to answer your questions is likely going to be the engineer that is/has designed the wall. Your additions are a significant factor that should be considered when planning the wall, not a change made after the fact. The calculations needed to give you a simple answer as far above my head and likely most of our members. However, for the engineer, it could be as simple as swapping from 50mm sleepers to 75mm in their design.

We need to think of the worst-case scenario here. Say you have a huge party and fill this seat out to capacity with 20 people. This is unlikely, but in the unlikely event that it happened, the additional 1.4 tonnes of weight multiplied by the counter-levered seat would be far more than what the engineer ever intended. They've engineered the wall to account for the retained static load of the soil behind the wall, not a variable force on the front of the wall as well. I would hazard a guess that your wall will non-longer comply with regulations as you've significantly modified it beyond the engineer's plans. 

Failing to have this seating worked into the design, the next best place to find information will be with the concrete sleeper manufacturer. If you were going with our concrete sleepers, I'd be happy to contact the manufacturer to find out more information on what modifications can be made.

Please let us know if you have any questions.

Mitchell

[StephaneDubois]

Hi Noyade

I think the Sleepers will be 100mm thick, but I’m still waiting for the engineering study to be completed.

yes, you are right the cost of the brackets is pretty high! There must be a more cost efficient alternative. 

haven’t spoken to my contractor yet but will do.