Re: Add a timespan to deadline using support hours
The duration type fields that take SLA into account are Actual Duration, Open Duration and if Configured at server settings "Planned Duration"
Unfortunately Open Duration is read only.
So you have two options, either set deadline = actual finish where actual start = reg;created and duration = 16 or set deadline = planned finish where planned start=reg;created and planned duration = 16
In any case you have to make sure that SLA is set before trying to do that thing or else the calculation will be 24x7 based.
To force Planned Duration to take SLA into account check "Planned Calculation uses Service Level for Servicecalls and Incidents, otherwise 24x7" on tab [Application] of General Settings on administrator console.