ceiling 56 of the gallery 20 ventilation of the gallery 20 can be provided by ventilators 58 also disposed in the ceiling 56 as can best be seen in fig1 and 3 through 6 the ceiling 56 forms an angled ceiling portion 60 at the upstream end of said gallery 20 the angled ceiling portion 60 defining a surface which is parallel to a downstream angled slab surface 62 comprising a surface portion of a v shaped trough 64 formed in upper slab surfaces of the sill 22 the bore 40 within which the strut 14 moves is formed normal to the angled ceiling portion 60 and the downstream angle slab surface 62 of the trough 64 and extends therethrough to receive the strut 14 the upstream angled slab surface 66 of the trough 64 is disposed at anangle of45 u00b0 to the horizontal in order that the strut 14 can move through the trough 64 the angled ceiling portion 60 is also located at an angle of 45 u00b0 to the horizontal the trough 64 is preferably provided with a flattened raceway 66 which can extend along the length of the concrete sill 22 and which can extend upstream of the trough 64 to communicate with the pocket 26 thereby forming a depression into which the gate 12 fits on full lowering of the gate 12 as is seen in fig6 a downstream notch 68 can also be provided relative to the trough 64 in order to accommodate the distal end of the gate 12 on full lowering thereof it will be understood by those of ordinary skill in the art that the upper slab surface of the concrete sill.