Why not use a navwalker to create your bootstrap menu?
Try https://github.com/wp-bootstrap/wp-bootstrap-navwalker
You can get the fixed-top to add to the menu
<nav class="navbar navbar-default navbar-fixed-top" temscope="itemscope" itemtype="http://schema.org/SiteNavigationElement" role="navigation">
<div class="container">
<div class="navbar-header">
<button type="button" class="navbar-toggle" data-toggle="collapse" data-target="#navbar">
<span class="sr-only">Toggle navigation</span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
</div>
<div class="collapse navbar-collapse" id="navbar">
<ul class="nav navbar-nav">
<?php
wp_nav_menu( array(
'menu' => 'primary',
'theme_location' => 'primary',
'depth' => 2,
'container' => 'div',
'container_class' => '',
'container_id' => 'collapse navbar-collapse',
'menu_class' => 'nav navbar-nav',
'fallback_cb' => 'wp_bootstrap_navwalker::fallback',
'walker' => new wp_bootstrap_navwalker())
);
?>
</ul>
</div>
</div>