sentences:  Then the slope of the soap film at any area of the cross section is directly proportional to the stress in the bar at the same point on its cross section.Simply when steel subjected or installed in a structures load distribution from slab to footing 
  Consider a steel element which will allow transfer load through it in a steel molecular are in stress may be normal or shear stress variation steel element trying to resist loads how much stress occurred in length of steel bar or structural element are drawn in contour lines
  
  While the membrane analogy allows the stress distribution on any cross section to be determined experimentally, it also allows the stress distribution on thin-walled, open cross sections to be determined by the same theoretical approach that describes the behavior of rectangular sections.Using the membrane analogy, any thin-walled cross section can be "stretched out" into a rectangle without affecting the stress distribution under torsion.