Construction Principle Animation:Prestressed Concrete Box Girder Lab

Without prestress, a 40 m reinforced concrete beam cracks under its own weight; more load means more, deeper cracks and a deeper sag.

Tendons dip near the soffit at midspan and rise to the centroid at the ends, giving the biggest upward push at midspan without cracking the ends. Try switching the profile to straight.

As force is applied stage by stage, the soffit goes into compression, midspan rises and the girder lifts off the falsework—its weight now carried by prestress and concrete together.

Prestress pre-compresses the soffit; traffic only uses up part of it, so the soffit never goes into tension—no cracks, and a fraction of the plain beam’s sag. Watch how much reserve is left under overload.

Strand relaxation, shrinkage and creep erode prestress over the first years. Design must include these losses so the soffit still won’t crack under full load after ten years.