Bootstrap navbar always in "hamburger mode"


hi there.
i am aiming to building an app for ios and android. i am using a standard bootstrap navbar and want it to always be in mobile-mode (only the “hamburger” (three lines) is visible and on click all li-elements are displayed underneath each other).

the iphone6 shows the li-elements in this ugly floating state. how can i make the navbar HAMBURGER ONLY??!

thanks in advance!!!