A bridge is a good simile to describe friendship because when you have any problems your friends will help you and support you, just like a bridge over troubled water.
But I will use an apple tree to describe friendship. When you are tired, you can take a rest under the apple tree, just like your friends will let you rest on their shoulders. When your friends are happy, I am sure that they will share their happiness with you. The apple tree is the same. It has many apples for you to eat. The apples are the happiness of your friends. If you don’t water the tree, it will die or doesn’t have the fantastic apples. If you don’t care for your friends, they won’t share their joy with you either. So take care of your friends. They give many things to you and help you much. They are the apple trees of your life.