Engineering 101 - how to calculate span strength

Forums › Home Workshop DIY › Engineering 101 - how to calculate span strength

mdf

3516 posts

Uber Geek

Trusted

#261377 24-Nov-2019 10:01

I am thinking about a plan for a (very) large workbench - 3m x 1.5m:

My original plan was to have 3 or 4 legs down the side of the long axis. Usually if I am in any doubt, I just add another brace.

However, I've now realised that under the workbench would be an absolutely perfect place to store full sheet stock of plywood, MDF etc. But I won't have enough room to manoeuvre full sheets in under the short axis, it will have to go in under the long axis. So to make this work, I could only have legs in the corners. Which means if this is a viable plan, I think I will need a reasonably chunky horizontal beam running between the legs.

What are the right engineering terms to google to work out the requirements for this beam? Is a horizontal beam still a "lintel", or is that just for doors and windows? It will need to be quite stiff to revent sagging. What is the right engineering terms for "stiffness"? I'm open to the possibility of using steel if required.

I'm still very much in the planning stage, but I'm currently thinking about laminating either solid wood or (more likely) LVL into an LVL benchtop, something like this. So the top should provide some stiffness too.

RunningMan

8960 posts

Uber Geek

#2359989 24-Nov-2019 10:18

Whatever you end up doing, remember that it's not just the static load that it has to support (like in a building). If you're hammering (or similar high dynamic loads) you don't want the bench to flex or bounce underneath whatever you are working on.

neb

11294 posts

Uber Geek

Trusted

Lifetime subscriber

#2360083 24-Nov-2019 14:05

Isn't the idea for workbenches to just overbuild them to the point where they'll support a tank driving over them? They're usually specified in terms of weight and quantity of materials used, not engineering calculations, e.g. 850 pounds, 600 screws.

freitasm

BDFL - Memuneh
79294 posts

Uber Geek

Administrator

ID Verified

Trusted

Geekzone

Lifetime subscriber

#2360085 24-Nov-2019 14:14

mdf:

However, I've now realised that under the workbench would be an absolutely perfect place to store full sheet stock of plywood, @MDF etc.

Why would you want to sleep under the workbench?

neb

11294 posts

Uber Geek

Trusted

Lifetime subscriber

#2360109 24-Nov-2019 15:14

Oh, another reason for linking to that video was to suggest alternative materials, the LVL you linked to is nearly $200 a panel while the timber in the video was $0. You could maybe get demolition timber like rafters and rip them down to what you need.

And build an 800 pound workbench :-).

neb

11294 posts

Uber Geek

Trusted

Lifetime subscriber

#2360111 24-Nov-2019 15:16

freitasm:
mdf:

However, I've now realised that under the workbench would be an absolutely perfect place to store full sheet stock of plywood, @MDF etc.

Why would you want to sleep under the workbench?

He'll have to when Mrs.MDF finds out how much he spent on the LVL :-).

evilengineer

466 posts

Ultimate Geek

#2360144 24-Nov-2019 17:38

mdf:

I am thinking about a plan for a (very) large workbench - 3m x 1.5m:

What are the right engineering terms to google to work out the requirements for this beam? Is a horizontal beam still a "lintel", or is that just for doors and windows? It will need to be quite stiff to revent sagging. What is the right engineering terms for "stiffness"? I'm open to the possibility of using steel if required.

I'm still very much in the planning stage, but I'm currently thinking about laminating either solid wood or (more likely) LVL into an LVL benchtop, something like this. So the top should provide some stiffness too.

You might be overthinking things, to be honest.

A beam is a beam is a beam. Whether it's called a lintel or a joist or a whatever is purely a result of the builder's tendency to give every piece of timber a different name depending on where it's located and what job it does.

A horizontal member that transfers a load by spanning between points of support is a beam. Simple as!

Anyway, if you really want to know the maths you should be searching for "Engineer's bending equations" and "beam theory".

I would suggest you "oversize" the front, back and sides to 140x45mm(say) Structural Grade timber or even 190x45mm if you want to get into park-a-tank-on-it territory. And provide an additional central beam to give three 3m long beams spaced at 750mm centres. Also connect the "beams" together with with blocking pieces (noggins/dwangs) at either 1/3rd or 1/4 points beneath the bench top.

evilengineer

466 posts

Ultimate Geek

#2360145 24-Nov-2019 17:42

neb: And build an 800 pound workbench :-).

An 800 pound workbench?

That's $1,600 at current exchange rates! 😮

Quic Broadband

Move to New Zealand's best fibre broadband service (affiliate link). Free setup code: R587125ERQ6VE. Note that to use Quic Broadband you must be comfortable with configuring your own router.

elpenguino

3424 posts

Uber Geek

#2360146 24-Nov-2019 17:43

Random idea:

Why not make centre struts that are removable. If you dont want to move large stock every 5 minutes then a removable strut will take the load during those dynamic events mentioned.

Since the forces are pretty much vertical any strut barely needs to attach to the workbench at all.

Most of the posters in this thread are just like chimpanzees on MDMA, full of feelings of bonhomie, joy, and optimism. Fred99 8/4/21

nickb800

2719 posts

Uber Geek

Trusted

#2360156 24-Nov-2019 18:12

As above, you can calculate a static load easily but allowing for dynamic loads like hammering will be much harder.

It's a fairly long span that you're proposing. Assuming it's up against a wall, can you put one or two legs along of the long edges? Then have regular ribs running along the short dimension to transfer loads from the unsupported long edge. Definitely glue/screw the top to the structural members, and consider glue/screwing another piece of mdf/ply to the underside of the structural members. This would stiffen it all up and aid transfer of load from the unsupported edge. I'd try for 190x45 as your beam.

SATTV

1648 posts

Uber Geek

ID Verified

#2360176 24-Nov-2019 19:50

My workbench is some old pallet racking cut to size, I think the bench itself is rated to 500KG

Mine is 3000l 900w and 1000H, I could not quite get a full sheet of gib as it is 900w but there might be 1200w versions available.

Cheap too, less than $100 for two ends, four rails, 2 sheets of MDF and all the clips needed.

John

I know enough to be dangerous

Bung

6488 posts

Uber Geek

Subscriber

#2360327 25-Nov-2019 09:15

Using it as a sheet product store raises a couple of points.
If you needed to move the bench you'd probably have to shift all the MDF etc
If the sheets were mixed the one you wanted would always be at the bottom leading back to 1st point.

geoffwnz

1593 posts

Uber Geek

ID Verified

#2360775 26-Nov-2019 09:19

Bung: Using it as a sheet product store raises a couple of points.
If you needed to move the bench you'd probably have to shift all the MDF etc
If the sheets were mixed the one you wanted would always be at the bottom leading back to 1st point.

That's just Murphy's Law of stacked storage though isn't it? The item you need will always be the one at the bottom.

MotorDrive Rallying

Nate001

645 posts

Ultimate Geek

#2360804 26-Nov-2019 10:04

Google "second moment of area of beams". You'll need tall beams to stiffen the table to reduce the bending moment in the material. As others have suggested its easy to calculate the static loads but the dynamic will be a bit harder. Laminating the table is doing this as you're increasing the "tallness" of the shape, but having taller beams will be more effective.

Assume worse case for the max load to be in the center and this will be the location of the max bending moment. The goal will be to keep the tension of the underside of the material under its max limits, as this will be the most likely failure mode.

Jeeves

301 posts

Ultimate Geek

#2360828 26-Nov-2019 10:28

Given it's so long, and workbenches naturally invite a lot of lateral loading (Ie: hand sawing), I'd think about some lateral bracing as well, otherwise you'll have wobbly table syndrome.