The　Hathaway　Bridge,　an　eastbound　and　westbound　bridge　pair,　is　located　in　Panama　City,　Florida.　Each　of　the　pair　consists　of　a　single-box　precast　concrete　segmental　girder　with　a　deck　width　of　24.4　m　(80　ft).　The　lengths　of　the　eastbound　and　westbound　bridges　are　1,031.43　and　1,162.93　m　(3,384　and　3,815　ft),　respectively.　To　date,　these　are　the　largest　single-cell　precast　segments　without　internal　struts　fabricated　in　the　United　States.　During　construction,　many　web　cracks　developed　in　the　external　anchorage　areas.　The　purpose　of　this　investigation　is　to　identify　the　main　causes　of　the　cracks　and　provide　some　design　recommendations　for　large　precast　concrete　segmental　box　girders.　First,　a　brief　description　of　the　bridge　is　given.　Then,　analytical　models　for　the　bridge,　posttensioning　forces,　and　construction　forces　are　presented.　The　analytical　results　show　that　the　external　longitudinal　posttensioning　forces　can　cause　significant　high-tensile　stresses　in　the　interior　face　of　the　web　around　the　anchorages.　The　information　presented　in　this　paper　can　assist　bridge　engineers　in　the　design　of　concrete　segmental　box　girder　bridges.　©　2014　American　Society　of　Civil　Engineers.