why does my workbench twist?

Question

I'm new to woodworking and recently built a simple workbench.

The tabletop is made of 3x6 lengths glued & screwed together, and I made a frame from 2x2 with a shelf at the bottom. The legs are fairly tall at 90cm. Everything is fixed using screws, atleast 2 for each join, and steel brackets also.

When I assembled everything I noticed that there is a fair bit of movement if you grab the tabletop on either end and twist it, I'm looking for ways to minimise this movement and also find out what I could do better next time.

My suspicion is that I've used wood that's too small / weak for the frame, but it seems very sturdy otherwise. What could I do to salvage this frame?

Note - since these pics were taken, I've also added 2" steel corner brackets to the insides of the corners at the top. This didn't seem to help with the movement much. Also, that janky support at the bottom front for the shelf has been redone, looked awful.

Answer 1

Triangulation is the answer here. The flex and twisting that you see is due to the fact that the rectangles formed by the legs, top and shelf supports are able to parallelogram easily. The long lever arms of the legs are easily able to distort the joints you created even with two screws.

There are a number of methods you could use to correct the problems.

1. You could use more of the 2x2 material to create angle members from one top corner of each of the rectangles to the opposite bottom corner. If properly secured at the ends the angle member can resist the racking that is currently happening. One angle member works for racking in both directions because the 2x2 material can work up in both a compression mode and an extension pull mode. You may want to avoid adding an angle brace across the front because it would impede the use of the shelf even though it would add additional bracing to the table.
2. You could cover some of the rectangle surfaces with a thin sheet material like plywood, Masonite or OSB. This would significantly reduce any racking that occurs in that rectangle. Of course you have to attach this material at multiple points around its periphery. Material on the back and sides should be more than sufficient. One suggestion would be to use peg board material for these sides as you could utilize those surfaces for hanging things.
3. Another solution that can be used is to use thin steel banding material to create the triangulation. Since such material is only effective in an extension stress mode you would need to apply two such bands on each rectangle surface as a large X across the corners. This material is often available in a roll that has holes at regular intervals along its length (a.k.a, plumbers strapping) that could be very easy to add to your existing structure by using short screws to attach it to the wood members in each corner.

With any of these ideas you want to make sure to use a framing square to ensure that the leg structures are at right angles to the top before securing the braces, panels or banding.

Answer 2

why does my workbench twist?

There are basically two or three ways you can make a joint rigid

Joinery

Maybe I'm wrong but it looks like you used butt joins held together with a single screw and some glue. Glue works best on joins where long grain is pressed against long grain. Glue generally doesn't work well on end-grain.

Only having a single screw means the joint is more likely to twist and rack.

The traditional way to join table parts is to use mortice and tenon joints, a quicker/easier alterative is to use two or three dowels per joint. I find lap joints are easy to cut and very strong and rigid when glued - they don't look very spiffy but that doesn't matter for a workbench.

(screws are there only to clamp glue until dry)

Obviously, this doesn't help with your existing structure but you might wantto consider this aspect next time you build a bench or table etc.

Bracing

Michael Karas covered this in his answer. This can make a huge difference. In your shoes I would add bracing to make the structure more rigid. Adding boards as an apron around the top may help if well secured to the legs.