Quartz countertop seam strength
Absolutely not! Although the seam has been sealed with an adhesive and coloring compound, it provides little flexural strength. Despite the back support, and with little to support the span between the left and right side of the countertop, it will easily break, probably with little more than body weight. Again, the "adhesive" is just a waterproof binding agent for colorant to fill the seam and is definitely NOT something to rely on to keep your countertop from breaking! I would at least add support from below using low-profile 4"X 6" angle steel on both sides: bolted to your cabinets and glued to "quartz".