The US National Gallery of Art will construct two new art wings and an outdoor sculpture terrace after securing US$30 (23.2m euro, £19.83m) in private donations.

The additional 12,260sq ft (1,139sq m) of exhibition space will be constructed within the current footprint of the East Building on Washington's National Mall.

Philanthropists who donated to the cause include: gallery president Victoria P. Sant and her husband Roger W. Sant; Mitchell Rales, a member of the gallery's board, and his wife Emily Rales; and David M. Rubenstein, co-chief executive of The Carlyle Group.

An outdoor sculpture terrace will measure 6,105sq ft and include stone paving, seating and tree planters.

Two Tower Galleries, which will be hexagonal in shape and primarily display modern art from the permanent collection, including paintings by Mark Rothko, will flank the terrace.

Galleries will close on a gradual basis from July to December this year, before construction begins in January 2014, lasting three years.