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Which post support for 90x90 cypress post over concrete for Merbau fence panel?

Hello Community,

I built a small screening with gate to divide an open area using Specrite 1800x900 alternating slate fence panel, pryda bolt down post support (one bolt) and 90x90 cypres dar post. M10 Ankascrews 100mm were used.

The width of the area is 2800mm intended height of screening is 1800mm. The surface is concrete. the post were 1700mm apart and fence panels were bolted on it. To minimise I have used two panels horizontally to make it 1800mm between two posts. I have noticed a sag in the post towards the panels, yes, they are heavy. Also, the posts have sway motion and wobble.

I am guessing it is because the pryda post support is not correct for this application. please suggest what kind of post support or bolts I should use to make them sturdy and rock solid for windy area.

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MitchellMc

Welcome to the Bunnings Workshop community Vikas S. It's wonderful to have you join us and many thanks for your question on attaching posts.

Even though bolt-down post supports like the Pryda are strong in terms of vertical load, they only provide minimal lateral (sideways) support. What’s happening in your case is that the base is securely fixed to the concrete, but you’ve effectively got 1800mm of unsupported post above it. When wind or the weight of those heavy merbau panels pushes against the post, the force applied higher up creates significant leverage at the base. That leverage causes the sway and wobble you’re seeing.

To properly resist lateral loads, posts are normally concreted directly into the ground. As a general rule, about one-third of the total post length should be buried. So if you need 2700mm above ground, you’d typically want around 900mm in the ground. Obviously, because you’re working over an existing concrete slab, that’s not an option here.

In situations where posts are bolt-down fixed to concrete, lateral stability needs to come from additional bracing rather than just the base. The most practical solution would be to tie the tops of the posts back to a solid structure. For example, you could run a 90x90 or even a 90x45 timber brace from the top of the post across to a nearby wall or bolt the post you'll be attaching the gate onto to the wall, and then connect the two tops of the posts. Better still, run a 90x90mm post horizontally across the top of all three posts and attach it with long landscape screws to the tops of the posts. That will significantly reduce movement by stopping the top of the post from flexing. Without adding that kind of bracing or embedding posts into the ground, it’s very difficult to make tall, heavy screening panels completely rigid in windy areas using bolt-down supports alone.

I've added the below rendering to assist.

Please let me know if you have any questions.

Mitchell

Vikas S

thanks for the render that really helped.
for the top brace i have to increase the height which will ruin the facade.
i am planning to keep the height at 1800 only. Will steel posts with base plate help here? Or may be use more posts and install the panels vertically instead of horizontal?

EricL

Hello @Vikas S

Have you considered cutting the posts down to 1710mm so that when you place the 90x90 post it will end up at 1800mm? It's one way to keep the height you are after and provide support at the same time. Will steel posts with a base plate help here? Because the Merbau panel is so heavy, I think you will still get unintentional sway from the structure. If given enough time it will either pull the bolts from the floor or the welding joints will give way.

Installing more post will not help as there is no support at the top to prevent the installation from swaying. Changing the direction of the Merbau panels will not help as the weight of the panels remains the same.

Let me call on our experienced members @Dave-1, @Nailbag and @Remarka6le for their recommendation.

If you need further assistance, please let us know.

Eric

Dave-1

Good Evening @Vikas S

Would you be able to install one other post, same height as your existing ones along the side of the retaining wall in line with the post there?

Firstly This way you could tie the rear post to the fence post (Diagonally and across the top) and Secondly you could go diagonally from the rear post to the post next to the gate. So you end up with a triangle at the top of the posts. The second choice is not necessary but will firm up the structure a fair bit.

Post anchor is what I have used in the past for a similar situation. Two fastenings to the concrete. You will need to space the post of the base via a spare piece of timber as you install the post is all.

Dave

Nailbag

Hi @Vikas S

Basically, what you have is a panel thats not only front heavy to the footings brackets, but a wind sock to some degree. The movement in the structure against the bracket bolts is causing the holes to elongate giving you that lean.

There are two things you could do to strengthen the structure. 1. Use angle brackets of the left post tied back to the concrete wall. 2. Build a short return fence off the right post. These two will provide a very strong solution long term and will prevent any further mobement.

Nailbag

Noyade

Hi @Vikas S

"Will steel posts with base plate help here?"

I would keep the wood and build serious custom brackets from 5mm steel plate with four large ankascrew positions.

Noyade

Second thought. Add stiffeners…

Miniature version with pipe…

Noyade

I don't know why I can't get that sentence between the photos.

😕

EricL

Hello @Noyade

I believe you can do this with the image menu when placing the photo in the post.

Eric

Dave-1

Evening @Noyade

yeah I have found it a little fiddly, but I cheat :)

I highlight the photo, then space, then return so even tho I cant see it I "should" be on the next line. Without that sequence I run into that samne type of issue you have described.

Dave

Noyade

I don't quite follow Dave.

The weird thing is when you go Edit - it looks fine.

Press Save - and it goes back to what you see above.

On my phone there is a huuuuuuge gap between photos.

MitchellMc

Hi @Noyade and @Dave-1,

The text is going to the right-hand side as the image is set to 'Float left'. Change it to 'Float centre', and the text will go between the images as I've now done for Noyade's post above.

Mitchell

Noyade

Got it! Thanks Mitch! 👍️

Dave-1

Morning @Noyade

I rarely use my phone for the webpage :) And yeah @MitchellMc will keepan eye out or it next time. Old habits die hard…. and now I want to watch the Christmas Movie….

Dave

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