Steel Poles

If you fill the PVC with sand, and you need to take it apart, you will have a nice beach, if you want it or not.
And yes, I have put sand in PVC before.

Not hard at all to build.
Everything can be found at any hardware store.
Pipe will work here as well as steel tubing.

[Like Ra]

Pipe will work here as well as steel tubing.

Speaking of pipes (PVC pipes). How do you fix them firmly in place without glue? Drill a hole through the pipe joints and insert bolts with a nuts?

Uhh, pipe is threaded water pipe. Sorry, I should be more clear.
Pipe = threaded steel water pipe.
Steel Tubing = round or square.
PVC = plastic tubing, pipe.

At lease that's what we call it here in nor-cal, USA.

Crap.....
Forgot to answer your question Ra,
Yes, you drill holes in the fittings and PVC pipe for 1/4" bolts or pins.
This way, all you need to do is pull the pins and everything breaks down for easy storage. There is a small problem with the breakdown gear. When assembled, it does move, wobble, creaks and cracks.
I like my things a little more ridged.

[Like Ra]

OK, so we mean exactly the same, good 😉 I was thinking about lockable pins, but still you will have this:

When assembled, it does move, wobble, creaks and cracks.

BTW, I've an idea... You can thread ropes through the pipes and tighten them according to the force directions. Just like what muscles and tendons do to the skeleton. That should make the whole construction more rigid.

Or, you can use tiedown straps. Then it is easer to explane.

[Like Ra]

tiedown straps

With ratchets. Even beter, indeed.