I received construction documents for an extensive residential remodel and looking at the engineering there's a glulam drop beam with a calculated member reaction that is 100% of allowed (3965 lbs @ 5 1/2"). No other member comes close to 100%. Does this have any practical implications? Is this common?
This means that the member is considered to be fully loaded for a failure mode which is being analyzed by that stress calculation.
Some things to (probably) keep in mind include:
Some conservatism likely exists in the analysis/design calculation. A member reported to be at 100% capacity is (probably) not actually at 100% capacity. This conservatism could include anything from assuming larger loads than actually exist, assuming smaller section sizes, assuming longer member spans, etc.
The load case used to arrive at the conclusion that this member is at 100% capacity (probably) considers a transient load of some sort (like wind, seismic, dynamic water pressure, etc.) that isn't acting on the structure at all times.
Structural members are usually checked for several different failure modes. An indication that a member is at 100% capacity for one failure mode (probably) doesn't mean that it is at 100% capacity for all failure modes.
Sometimes a member shown to be at 100% capacity based on the stress/design calculation may not be at 100% of it's strength capacity. Many times structural members will be controlled by their serviceability requirements instead of their strength requirements.
For a light weight design (e.g. an aircraft) you would try to achieve 100% of the allowable loads for every part of the structure, under some operating condition (not necessarily the same condition for every part, or course). But in building design, other considerations may be more important - for example using consistent sized components could minimize cost (by ordering in bulk) and/or simplify the construction. If a set of beams are all specified as the same size, you can't install the "wrong size" in some particular location.
Common components like beams are manufactured in a standard range of sizes. If the full range of different sizes in the relevant building codes and standards are not available locally "off the shelf", it may be cheaper and/or quicker to use over-sized beams rather than find a supplier for the minimum size that is actually required. Of course that will reduce the load factors in the oversized parts.
Also, the stiffness of some parts of the may be more important than their strength. For example it would be a bad idea to design a floor that was perfectly safe to carry the expected loads, but which flexed up and down when you walked across the room.