Creating complex shapes with box2d

I am trying to create a body in box2d which looks like this:

http://www.tobyjones.com/3d/3d_rec.jpg

I was going to create a horizontal rectangle and add vertical rectangles to each e开发者_如何学Cnd of the horiztontal rectangle.

I want to create this as a single body and so I was under the impression I should create 3 shapes and attach these to the body using a fixture, however I cant see how I'm surposed to position the shapes relative to the body.

So my question is, how do i position each shape within the body using fixtures?

I believe you also need to use the CreateShape() method for each shape you add:

     bodyDef = new b2BodyDef();

     body = world.CreateBody(bodyDef);

     boxDef = new b2PolygonShape();
     boxDef.SetAsBox(5,1);

     body.CreateShape(boxDef);

     boxDef.SetAsBox( 1, 5, b2Vec2( -5,5), 0 );

     body.CreateShape(boxDef);

     boxDef.SetAsBox( 1, 5, b2Vec2( 5,-5), 0 );

     body.CreateShape(boxDef);

//middle shape
polygonShape.SetAsBox( 5, 1 );

//left end shape
polygonShape.SetAsBox( 1, 5, b2Vec2(-5,-5), 0 );

//right end shape
polygonShape.SetAsBox( 1, 5, b2Vec2( 5,-5), 0 );