New York City’s top structural engineer Silvian Marcus — who once made a video that went viral explaining how 432 Park Ave., the tallest residential structure in the Western Hemisphere, stays upright — is also a real estate investor in his own right.

The man who has had a hand in creating the city’s trophy towers, including supertall billionaire buildings like 432 Park Ave. and 157 W. 57th St., has just sold a prewar condo at the Pythian for $1.45 million, according to city property records.

The one-bedroom, two-bathroom unit, at 135 W. 70th St., is 953 square feet and closed for its full asking price.

The apartment has large-scale rooms, 13-foot ceilings, tall windows and lots of light. There’s also a chef’s kitchen with a breakfast bar, a master suite with a dressing area and an “oversized” bathroom with a soaking tub and custom cabinets, along with a second marble bathroom, according to the listing.

The building was designed by Thomas W. Lamb — who designed many New York theaters — in 1926.

Building amenities include a concierge, bike room, lounge and storage room. The listing brokers were Maria Pashby and Joanna Pashby, of Brown Harris Stevens.