As the title says:
total_weight column in
sales_flat_shipment is always null.
Can this be a bug?
I have just made investigation.
total_weight field is created but is not used in the code.
Using search I did not found setting or getting value from array with key
Also there no
setTotalWeight functions usage.
So I can say that this functionality is not used anymore.
The only place where it can be used is to save TotalWeight value from FedEx api responses.