Can I use 2x6 LVLs 12 inches on center to span 7 1/2 feet to support a live load?
The interior wall they'll rest on is brick, the exterior wall is frame 2x4s(mostly windows) with 4x4 posts on a concrete pad (old sunporch footers).
Would the weight of the LVLs squishle the exterior enclosed sunporch wall or the old footers? I'm in Chicago, and know that the cement pad is original to the house (1920s bungalow) and hence NOT the required 42 inches in depth.
The weight of an LVL is not going to squash anything. However, the weight the LVL may support could certainly overload the LVL or the supports. In order to determine the required size of the LVL and supports , you need to compute the sum of the dead load (weight of framing) and the live load (typically code driven). To do this, you need to compute the tributary area of ALL load bearing areas above the beam, which may include portions of floors above the beam, or walls. Then you multiply the tributary area by the dead and live load values per square foot.
This type of computation is typically done by either an architect, engineer, or knowledgeable contractor. If you do this type of computation DIY, you may have trouble getting a permit, especially in Chicago, which as I recall from my days in the midwest is pretty rigorous about permits.