In the land of ten gallon tanks, a 100g is huge. But in reality it isn't that big. Many are made without the middle brace. If yours has no trace of silicone there, it was made without one. If there is old silicone there, the thickness should match the thickness of the tanks glass.
Btw, if your tank has a plastic trim frame around the top without a cut out to allow for glass to glass contact, it does not need a brace. You only will need a piece of glass in the middle to support the lids. It will not need to be siliconed in.
Btw, if your tank has a plastic trim frame around the top without a cut out to allow for glass to glass contact, it does not need a brace. You only will need a piece of glass in the middle to support the lids. It will not need to be siliconed in.