Question: I'm having a problem with Physics! I have imported an object which a bridge from 3ds max to unity 5 ,but the problem is that whenever I want to walk through on it ,I just fall down. It's like there's nothing called BRIDGE! I know there must be a problem about Physics. But how to fix it ?! 

[![enter image description here](https://i.stack.imgur.com/1B7KP.png)](https://i.stack.imgur.com/1B7KP.png)

Answer: 
When importing a model into Unity it does not have any Collider.

Even though using auto-generated "MeshColliders" is an option  

I **highly recommend** **not to use them**.  

You never need such highly detailed colliders in a game.  

Instead, you have to

Add the colliders manually
==========================

**Here is a little step by step guide**

This is the imported model (made with blender) without any collider whatsoever
[![enter image description here](https://i.stack.imgur.com/aInbz.png)](https://i.stack.imgur.com/aInbz.png)

### First thing to do is add a new box collider to the components

as shown in this gif
[![enter image description here](https://i.stack.imgur.com/rWSQQ.gif)](https://i.stack.imgur.com/rWSQQ.gif)

### The next thing to do is scaling the box collider

by either entering the values manually or using the drag/GUI version like I do in the gif below
[![enter image description here](https://i.stack.imgur.com/PZaUa.gif)](https://i.stack.imgur.com/PZaUa.gif)

*(you probably know that already but the mode can be changed to orthographic by licking the small cube in between the cones in the upper right)*

### If necessary you can add as many "detail" collider as you want

by repeating the steps. Sometimes sphere or capsule colliders might fit better but keep in mind that they have a higher resolution!  

In this case i added another box collider representing the upper part of the car:
[![enter image description here](https://i.stack.imgur.com/XouuD.gif)](https://i.stack.imgur.com/XouuD.gif)

Thats all you need to make things collide with your object
----------------------------------------------------------

[![enter image description here](https://i.stack.imgur.com/tnlXj.gif)](https://i.stack.imgur.com/tnlXj.gif)
Right now you only have a MeshFilter, MeshRenderer, and Animator component attached. You walk through it because the mesh is only being rendered. In order to add collision, add a [MeshCollider](http://docs.unity3d.com/ScriptReference/MeshCollider.html) component to it.

Joe offers another way to do it with box colliders. There is a bit to know about both methods:

* Box colliders will take more time to set up (rather than just adding a
component) and will have less precision. They will have faster performance.
* Mesh colliders do not collide well with other mesh colliders.
However, this is often not an issue. If a mesh collider has the
"convex" checkbox checked then it will collide just fine with other
mesh colliders. Small items and any convex-shaped meshes should have
"convex" checked.

For something like your bridge.. it depends on your game. If you are doing a top-down game with limited mobility (no jumping) then box colliders may work for you. If you are making a first-person game then I would strongly suggest mesh colliders. If you decide to use box colliders then you must set them up carefully. Otherwise, physics may not match up with what the player sees!

Do not worry about performance at this time. Worry about it later *IF* it becomes an issue. With the physics upgrade in Unity 5, it probably won't be.