Doe's anyone know how to place the girders, every time I try to place one it doesn't appear on screen, I've tried using the Space bar and the left mouse button.

I assume you're talking about Worms2, since it's been a while since the DOS (Worms) game came out. Use the arrow keys to rotate the and change the size of the piece, and left click to place it. you may be experiencing difficulties due to the fact that there are limitations and restrictions on where girders can be placed. i.e. they must be a certain distance from the ground and no higher than a certain elevation. The girder restrictions can be turned off in the game options screen.

The girders in the original Worms game never worked for me.

WICKED CHEAT: Disable the shotgun in the Weapon Options menu. When it's your turn, hold down F3 and F5 and fire. A swath of destruction will fly out of your gun and destroy everything in a huge arc that expands as it flies across the screen. It's the shotgun from Hell! However, be warned. If this cheat is used too many times, your computer will freeze up.
Also, if you do it right, you can use the super shotgun as many times as you want. You get two shots with the shotgun. For your first shot (With the shotgun from hell) fire so that you just barely injure yourself. Don't do any serious damage, just knock yourself down by a couple of points. If you do it right, you get an infinite number of turns armed with the shotgun from hell. Of course, you don't need an infinite number because four or five shots will kill everything on the screen!

Does anyone play the original game anymore? I'd love to know if the super-shotgun trick worked for anyone else. It's rather fun seeing that swath of destruction and it really surprises the people you're playing against too!