I solved the interpretation of the uniform distribution load using the following code.
class Top(SubDomain):
def inside(self, x, on_boundary):
return near(x[1], W) and on_boundary
w = 4e3
top = Top()
top.mark(boundaries, 1)
l = dot(Constant((0, -w)), v)*ds(1)
However, this code does not apply to the code you gave me with different solve format. I guess the solve format I used is solve (a==l, u, bc), but the solve format in the code you gave me is a matrix. I want to find a way to solve this problem.