I believe the SOLIDWORKS Corp Knowledge Base covers this in Solution S-020720.:
The combination of external load and bolt preload are not as simple as 5000 + 10000/4 = 7500.
The behavior of bolted assembly with or without preload is very different. When there is no preload, there is no clamping force on the joint. The joint gets separated easily (joint failure) and the force applied on the bolt increased rapidly with the external force. However, when preload is applied, there is a clamping load which equals to the preload value applied on the joints to compress them together. The external load has to overcome the clamping force first in order to separate the joints. The increase of the bolt force is very small compared to the joint without preload. Analytically, because of the contact involved, it is a nonlinear process. It is not a simple superposition.
The nominal stiffness of the joint (Km) changes as the contact area changes though the bolt stiffness (Kb) is the same. That's why the ratio of external force carried by the bolt is very different in the two study scenarios. The stiffer the jointed parts, the less the bolt force increased on the preload.
For a more detailed explanation please have a look at this webpage. http://www.boltscience.com/pages/basics2.htm
So, in short, some of the force transmitted by the bolted joint is through the clamped plates themselves and not down the bolt alone.
Thank you for your greatly appreciated answer.
Just a little question: what does 'Solution S-020720' mean?
As others have mentioned, the force which the bolt experiences is a fraction of the applied load, so long as there is sufficient pre load to keep the joint clamped together.
The bolt force is :-
Fb = Fpl + C*Fa
Fb is the bolt force,
Fpl is the pre load force,
C is the joint constant,
Fa is the applied load.
The joint constant, C, in its simplest form is:-
C = Kb/(Kb+Km)
Kb is the axial bolt stiffness,
Km is the axial plate stiffness.
There are a number of other influencing factors to the joint constant but the above is a reasonable approximation.
Refs. Shigley's Mechanical Engineering Design 8th Edition
VDI 2230 - Systematic calculation of high duty bolted joints Joints with one cylindrical bolt
Online Bolt Stess Calculator - https://www.digitool.org