It seems, that Bootstrap doesn't provide an instrument to implement such a mixed fixed-fluid layout. One has to use a workaroud like this (found here, see also http://jsfiddle.net/kizu/UUzE9/):
CSS
#main {
min-width: 500px;
padding: 0 500px 0 0;
white-space: nowrap;
overflow: hidden;
}
.main-column {
position: relative;
display: inline-block;
vertical-align: top;
}
#main-column-left {
width: 50%;
background: #00FFB4;
}
#main-column-middle {
width: 500px;
background: #434343;
}
#main-column-right {
width: 50%;
background: #FF9000;
}
.content {
white-space: normal;
}
HTML
<div id="main">
<div class="main-column" id="main-column-left"><div class="content">
Lorem ipsum dolor sit amet...
</div></div>
<div class="main-column" id="main-column-middle"><div class="content">
Lorem ipsum dolor sit amet...
</div></div>
<div class="main-column" id="main-column-right"><div class="content">
Lorem ipsum dolor sit amet...
</div></div>
</div>
But it's not a solution. So if someone knows a bootstrap conform way, please feel free to post it.