This method is very easy and allows you to discretely connect two parts only at the bolt locations. It also allows you to estimate bolt loads at each location. This method does not address the issue of bolt prying in the joint.
It has been observed from testing that most preloaded bolted joints will have good contact around an area of about two bolt diameters at each bolt. In between bolts there may be contact but actual stiffness will depend on many, sometimes uncontrollable, parameters such as surface finish, flatness and friction. It is generally more conservative to ignore contact between bolts unless some techniques are used to ensure good contact. Common techniques may include the use of Locktite, glue or oil between the interface surfaces to fill the small voids. If these techniques are used you may be able to justify ignoring the bolted connection (see Ignore Bolts).
To use spot-welds as bolts you will need to model a small gap between the interface surface either by assembling with mate-offset or creating a feature to ensure a gap. The gap should not be greater than 10% of the bolt diameter. Next fill in or remove the boltholes in the parts. Add a datum-point at the center of each bolt on one of the parts. Defined a Spot-Weld between the two interface surfaces with a diameter equal to two bolt diameters at each bolt location.
The very local stresses around the bolt locations will not be very accurate however the stress and deflection away from the local bolts areas should be good. Spot welds create beam elements between the parts. Shear and tensile loads in these beams may be used to estimate the load carried by each bolt.
