fix body when bootstrap navbar collapse

<div class="navbar navbar-inverse navbar-fixed-top" role="navigation">
<div class="container">
    <div class="navbar-header">
        <button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-collapse">
            <span class="sr-only"></span>
            <span class="icon-bar"></span>
            <span class="icon-bar"></span>
            <span class="icon-bar"></span>
        </button>
    </div>
    <div class="navbar-collapse collapse">
        <ul class="nav navbar-nav">
            <li><a href="#about">                  
                About
            </a></li>
            <li><a href="#contact">                  
                Contact
            </a></li>                
        </ul>
    </div><!--/.nav-collapse -->
</div>

The above is the code of bootstrap navbar header which collapses when screen size decreases. Could anyone help me to make the body position fixed only when the navbar header dropdown is open.

Thanks.