SO affix example

HTML

<link rel="stylesheet" href="http://getbootstrap.com/dist/css/bootstrap.css">
<script src="http://getbootstrap.com/dist/js/bootstrap.js"></script>
<body >
<div class="row" align="center">
    <div class="image"></div>
</div>
<div id="nav-wrapper">
    <div id="nav" class="navbar" >
        <div class="navbar-inner">
             <a class="btn btn-navbar" data-toggle="collapse" data-target=".nav-collapse">
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </a>
			<div class="brand"><a href="index.html" ></a></div>
            <div class="nav-collapse collapse">
                <ul class="nav">
                    <li><a href="#a">Aaaa</a></li>
					          <li><a href="#b">Bbbb</a></li>
				          	<li><a href="#c">Ccccc</a></li>
                </ul>
            </div>
        </div> <!-- navbar-inner -->
    </div> <!-- navbar -->
</div>
<div class="container">
    <div class="span3">
        <h2>some lorem ipsum for scrolling:</h2>
       Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.
       Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur...

CSS

#nav-wrapper.affix {
    top: 0;
    width: 100%
}

.image{height:300px; width:500px; background-color:grey;}


#nav > .navbar-inner {
    border-left: 0;
    border-right: 0;
    border-radius: 0;
    -webkit-border-radius: 0;
    -moz-border-radius: 0;
    -o-border-radius: 0;
}