edit: As lots of people seem to want to do this, I have written up a short guide with a more general use case here https://www.atlascode.com/bootstrap-fixed-width-sidebars/. Hope it helps.
The bootstrap3 grid system supports row nesting which allows you to adjust the root row to allow fixed width side menus.
You need to put in a padding-left on the root row, then have a child row which contains your normal grid layout elements.
Here is how I usually do this
http://jsfiddle.net/u9gjjebj/
html
<div class="container">
<div class="row">
<div class="col-fixed-240">Fixed 240px</div>
<div class="col-fixed-160">Fixed 160px</div>
<div class="col-md-12 col-offset-400">
<div class="row">
Standard grid system content here
</div>
</div>
</div>
</div>
css
.col-fixed-240{
width:240px;
background:red;
position:fixed;
height:100%;
z-index:1;
}
.col-fixed-160{
margin-left:240px;
width:160px;
background:blue;
position:fixed;
height:100%;
z-index:1;
}
.col-offset-400{
padding-left:415px;
z-index:0;
}