Crossings – Live Loads

Many questions are asked about API 1102 regarding wheel load cyclic stresses on highways and railroads.  Below are two most common that are frequently asked.

Highway Cyclic Stresses

Live highway load, w, the load due to the wheel load at the highway surface.  The load from only one wheel set needs to be considered.  An axle is considered to have two-wheel sets.  Design wheel load should be Maximum from a single axle:

 ·       Ps - Recommended default is 24 Kips per axle, or 12 Kips per wheel set.

·       Maximum from a tandem axle, Pt.  Recommended default is 20 Kips per axle, or 10 Kips per wheel set.                                                             

·       w (design surface pressure) = P/Ap, where

Therefore, the recommended defaults are 83.3 psi for a single axle and 69.4 psi for a tandem axle.   

Whether a single axle or a tandem axle loading is more critical is a function of the carrier pipe diameter, the depth of burial and whether the road surface is flexible pavement (Blacktop), no pavement (Open Right of Way)  or rigid pavement (Concrete).

Railroad Cyclic Stresses

Live rail load, w, pounds per sq inch, is the load applied at the surface of the crossing.  It is assumed that the load is evenly distributed over an area that is 8’x 20’.The recommended default is 80,000 pounds (80 Kips) per axle. 

 The cyclic, longitudinal stress due to railroad loading, ΔSLr= KLrGLrNLFiw, where:

 ·       KLr= stiffness factor for cyclic, longitudinal stress from rail load, as a function of twand Er, the resilient modulus of the soil.

·       GLr= geometry factor for cyclic, circumferential stress from rail load, a function of D and carrier pipe burial depth.

·       NL= railroad single or double track factor, = 1 if single track, a       function of D and carrier pipe burial depth if double track.

·       Fi= impact factor.

·       w =  surface pressure in psi.

Therefore, at the junction of two rail cars (four axles), 320,000 pounds is distributed evenly over the 8' x 20' area.  This default value reduces to 13.9 psi.  If a different loading is known to be the maximum, then it should be used.