Assigning a concentrated load to a specific point in the boundary

Hi everyone,
I am a beginner in Julia and I want to assign a concentrated load pointed downwards into the middle point of the boundary of my model. Unfortunately, this code gives me some results just when my tag used is a line (1D), not a point such as in this case. The code works but the stress values are 0.

I really appreciate your help.

using Gridap 
using GridapGmsh
import gmsh

Γ1 = BoundaryTriangulation(model, tags="upper middle point")
dΓ1 = Measure(Γ1,degree)
t1(y) = VectorValue(0.0,-100.0)
l(v) = ∫( t1 ⊙ v)*dΓ1 
op = AffineFEOperator(a,l,U,V)
uh = solve(op)    
sigma_num = evaluate(σ∘ε(uh), Point(3.0, -5.0))