A.Rachel Smith responds: The proper way to attach the bottom chord to the wall below depends on a few things. If the interior basement wall is load- bearing and the truss is using it as an intermediate support, the truss bottom chord must be directly attached to the wall, no matter what their relative orientations are. Check the truss design drawing from the truss manufacturer for information on the location and size of the required bearing points.
If the interior basement wall is non-load-bearing and perpendicular to the truss, you can leave a gap between the truss and the wall. However, for the sake of the wall's lateral stability, you may want to have some form of attachment, like a slotted metal anchor at the top of the partition wall, that will allow for vertical movement of the truss. If the truss is parallel and off center to the non-load-bearing wall, you can block the truss space over the wall and make an attachment. But there's no point in doing that if you plan to leave a gap, unless you're installing the slotted anchors.
Supposedly, the gap between the bottom chord and the wall is to allow for vertical movements of the floor (or roof) system and to avoid creating an "unintentional" bearing wall below. There are a lot of variables that affect how much load the truss will transmit unintentionally to the wall. It goes back to the old engineering adage "Stiffness attracts load," so the less stiff the support and the truss-bearing location, the less load it will pick up.
The upshot is that in most instances the gap is unnecessary. In fact, in the case of a basement partition wall, a direct attachment is preferable because it may reduce potential floor vibration problems.
Rachel Smith is the director of technical education at the Wood Truss Council of America.