how can i get a value form combobox GUI?

Hi all 

I wrote this code for combobox. However when i change the list the results not.

I expect when this list's value changes, get the area and moi for selected beam sections.

Could you please help me?

hwtk::labelframe $w.lf3 -text 'Defined List Only'
hwtk::combobox $w.lf3.cb -values $beam_list -help 'BEAM SECTIONS' -textvariable beam_sect

switch -regexp -- $beam_sect {
   I {
      set Inertia [hm_getvalue beamsects name=I_beam dataname=results_Icentroid0]
      set dim_1 [hm_getvalue beamsects name=I_beam dataname=beamsect_dim1]
      set dim_2 [hm_getvalue beamsects name=I_beam dataname=beamsect_dim2]
      set dim_3 [hm_getvalue beamsects name=I_beam dataname=beamsect_dim3]
      set dim_4 [hm_getvalue beamsects name=I_beam dataname=beamsect_dim4]
      set dim_5 [hm_getvalue beamsects name=I_beam dataname=beamsect_dim5]
      set dim_6 [hm_getvalue beamsects name=I_beam dataname=beamsect_dim6]
      set aera  [hm_getvalue beamsects name=I_beam dataname=results_area]
      set Icog [hm_getvalue beamsects name=I_beam dataname=results_centroid0]
   }
   C {
      set Inertia [hm_getvalue beamsects name=C_beam dataname=results_Icentroid0]
      set dim_1 [hm_getvalue beamsects name=C_beam dataname=beamsect_dim1]
      set dim_2 [hm_getvalue beamsects name=C_beam dataname=beamsect_dim2]
      set dim_3 [hm_getvalue beamsects name=C_beam dataname=beamsect_dim3]
      set dim_4 [hm_getvalue beamsects name=C_beam dataname=beamsect_dim4]
      set dim_5 [hm_getvalue beamsects name=C_beam dataname=beamsect_dim5]
      set dim_6 [hm_getvalue beamsects name=C_beam dataname=beamsect_dim6]
      set aera  [hm_getvalue beamsects name=C_beam dataname=results_area]
      set Icog [hm_getvalue beamsects name=C_beam dataname=results_centroid0]
   }