# Calculating Bending/Deflection of uprights



## VaguelyLost (9 Jan 2017)

Hi all,

I have been searching the forum/internet to find an answer but I cannot. At least one that is obvious to me as an answer.

I am designing a desk top for a sit/stand desk and I would like to incorporate a custom mount for 6 screens. I am thinking a simple L shape frame with the displays mounting to the uprights ( L<- Here ) and the horizontal hidden under the desk top.

What I don't know is how to calculate the dimensions of the lumber I should use to take the weigh of the screens without bowing forwards. The screens weight 4kg each and would be arranged in 2 rows of three.
My thoughts are to use through dovetail joints.

Any help is greatly appreciated.

C


----------



## nev (9 Jan 2017)

Hello and welcome.
Which timber etc would probably come into the equation, but as an example we had an old dressing table with a heavy mirror on two upright of only an inch square of some cheap unidentified hardwood, but they were just vertical rods fixed to the back of the unit. Putting all your weight and stress into a dovetail at 90deg, I would guess that the narrowest part of the dovetail would have to be the narrowest your chosen timber would need to be, if that makes sense?
There are some more equations at the end of the sagulator page if any of them may help?
http://www.woodbin.com/calcs/sagulator/


----------



## VaguelyLost (9 Jan 2017)

Thanks Nev,
The sagulator is a great tool. I have some left over 44x44 Spruce that I was thinking of using. I could also add a small brace to the jointed corners and simply use box joints. There will be 3 uprights sharing a load of 35kg including the screens, wood and lever effect (whatever the term is) as the are mounted proud of the frame

It looks like this will easily do the job.

FYI: This is the unit I am adding the top to: FURNA E2 ELECTRIC STANDING DESK FRAME - SIT STAND (cannot post links)
You may set the width of the frame to hold a desk with minimum size of 110cm x 60cm up to a maximum size of 160cm x 80cm with a total lift capacity of 120KG.

Thanks again for the help.
C


----------



## Shultzy (22 Jan 2017)

You need to be careful as the table's feet are longer at the front to withstand the pressure exerted at the front of the desk. Anchoring the screens to the rear might make the table unbalanced if nudged from the front. I would make sure the screens are as far forward as possible towards the middle of the table.


----------



## paulrockliffe (23 Jan 2017)

It's not a straightforward calculation because the crucial dimension isn't the weight of the monitors, it's the horizontal difference between the monitors centre of gravity and the centre of the supporting timber. that dimension is what gives the leverage for the monitor to bow the timber. If you can make that dimension zero, by recessing the monitors into the frame, then the monitors will exert no bending force on the timber, the timber will be loaded purely in compression.

I would start by considering how far away from the eyes the monitor needs to be and where that leaves the frame and then work from there. If it's a deep desk and the monitors must come forwards then I would consider a few options:

1. Can you mount the monitors to a wall instead? It's a lot of weight to try to balance on a desk anyway. If you built a matching wooden frame that attached to the wall that would float above the desk that would look very good, but also make this problem go away.

2. Can you use steel for the structure and hide it in wood? You'll get a much stronger and slimmer design that way.

3. Can you add a couple of architectural supports on the diagonal from the frame forwards towards the corner of the desk? Alternatively if the monitors are forward into the desk you may consider that to sort out the aesthetics from the side you need some sort of shielding panel that would provide structural support anyway. You could add a couple of supports at the edges under the front edge of your monitors too.

A few other thoughts:

Are you mounting the monitors flat in relation to each other? Personally I find they work much better if the outer monitors are angled towards you, so they're flat when you turn your head to look at them.

Have you considered the screen bezels? The smaller these are the better the displays fit together, the better the aesthetics. If you're building a custom frame for them I would consider removing the plastic surrounds to let them sit a bit closer. But you need to consider this at the design stage as you'll want monitors that don't have buttons on the front and that have been designed to have a thin bezel otherwise it won't be practical to remove the outer frame due to the design of the rear frame.

Have you looked at the effective resolution of the 6 screens and compared it to a large 4K display/TV? Looking at 6 1080 displays that covers about 45" diagonal won't be much different to one large 4K panel would it?


----------



## VaguelyLost (16 Feb 2017)

Shultzy":2z7dbwj7 said:


> You need to be careful as the table's feet are longer at the front to withstand the pressure exerted at the front of the desk. Anchoring the screens to the rear might make the table unbalanced if nudged from the front. I would make sure the screens are as far forward as possible towards the middle of the table.



Apologies for the delay in replying. I have been doing a little testing and so far stability is not an issue. However I have considered building in recess towards the front of the desk if ballast is required for balancing.


----------



## VaguelyLost (16 Feb 2017)

paulrockliffe":2uk72mlq said:


> It's not a straightforward calculation because the crucial dimension isn't the weight of the monitors, it's the horizontal difference between the monitors centre of gravity and the centre of the supporting timber. that dimension is what gives the leverage for the monitor to bow the timber. If you can make that dimension zero, by recessing the monitors into the frame, then the monitors will exert no bending force on the timber, the timber will be loaded purely in compression.
> 
> I would start by considering how far away from the eyes the monitor needs to be and where that leaves the frame and then work from there. If it's a deep desk and the monitors must come forwards then I would consider a few options:
> 
> ...




Thanks Paul.

I cannot remove any bending force but the monitors will be as far back as possible. To respond to each of your points:

1: This is not really an option as the unit will not be against a wall.

2: This is an option but if I can achieve the same result with wood, that would be preferred as I already have quite a bit on the rack.

3: Absolutely, this is already in my design. Effectively I am making a triangular box like / U shaped frame (open top and front). As the outer displays will be angled, I can use some of the space behind those to add the diagonal support.

As for Bezels, and 4K. I am already using a 6 display setup and already have the displays I will use. I have 2x 5K on another workstation and I honestly find the productivity on the 6 individual 1080 displays much greater than on a single or dual higher resolution monitor setup. My goal is to stick with my current setup and when 21-25inch 4K's get reasonable, upgrade to 6x 4k's. The goal of this project is to convert my existing setup to a sit/standing solution.

Thanks again for the suggestions.
C


----------

