"“When the walls break,” the Number Man said, “one million, seven hundred and thirty thousand tonnes of steel are going to drop on our heads.” [...] Alexandria flew forward. She caught the shelf of steel, concrete and granite. Buying time, even as the slab continued to crack and break down where the stress of her holding it warred with the sheer weight and lack of support in other spaces."
I'm going to be low-balling this by quite a bit. I haven't reached this chapter, but given the other descriptions of this I am lead to believe it is a very large building, holding several heroes many of whom can fly. Despite this, I will low-ball the distance the shelf fell to just one story, and then low-ball it further by subtracting Alexandria's supposed height.
Height of One Story - 3 m
Height of Alexandria - At least 1.72 m (described as "tall and athletic" by Taylor, who is between 5 foot 7 and 5 foot 8)
Ceiling Weight - 1,730,000 tonnes or 1,730,000,000 kg
3-1.72 = 1.28 m
√(2*9.8*1.28) = 5.01 m/s
KE = (0.5)(m)(v^2) = (0.5)(1,730,000,000)(5.01^2) = 21,710,000,000 J
Results
Alexandria Catches the Roof - 5.19 Tons, Large Building level
Should be noted this is nowhere near her maximum strength. She only failed to lift the roof because the roof collapsed due to lack of support. She no-selled it falling on her without issue and later practically swam out of it.