he U.S. government would begin collecting contemporary American art on a large scale if the National Art Bank Act, introduced by Sen. Harrison Williams (D-N.J.), gains the necessary support for passage. Sen. Claiborne Pell (D-R.I.), the cultural powerhouse of the Senate, has already signed on as cosponsor.

Based on the highly successful Canadian model, the Art Bank would help living American artists by buying their work at fair market value and making them available on loan to federal buildings all over the country, including court houses, post offices, etc. Works would also be available for loan to museums.

According to the bill, the Art Bank would be set up under the aegis of the National Endowment for the Arts, but with its own director and small staff. The director would appoint ad hoc juries to select works for purchase from artists and dealers all over the country. Criteria for purchase would be "quality" and "helping artists not well known to the public."

The bill calls for a $2-million appropriation for fiscal year '79, $3 million in 1980, and $4 million in 1981. The Canadian Art Bank has an annual appropriation of $1 million.

Once acquired, the art work would be available to federal facilities, but also to state and local governments, nonprofit institutions and private corporations, all of which would have to pay a leasing fee and guarantee public display and adequate protection.

In addition to collecting, the bank could also organize traveling expeditions and would be obliged to have state an d local governments set up their own art banks.

One new provision in the bill provides that artists could buy back their work after a reasonable time. This would permit an artist whose work appreciated in value to buy it back at the original cost, plus 50 percent of the increase in fair market value.

Another art-related bill, "The Federal Buildings Artistic Enhancement Act," will be introduced today by John H. Chafee (R-R.I.), with Daniel P. Moynihan (D-N.Y.) as principal cosponsor. This bill would give statutory authority to the General Services Adminstration's one-half percent for arts program, expenditures for which are now up to the discretion of the GSA adminstrator. It would also set up circulating exhibitions for federal buildings.