Washington is about to receive its biggest dose of architectural corn since Benjamin Latrobe stuck those corncob capitals atop the columns in the Capitol. It is a giant naval memorial arch near Pennsylvania Avenue, the plans for which were unveiled this week before a generally admiring Commission of Fine Arts.

Hand in hand with the Pennsylvania Avenue Development Corp., the Navy has turned its high-tech arsenal full upon contemporary architecture. The new arch may turn out to be quite a triumph, like Latrobe's capitals, but it has a long way to go. As designed by the New York firm of Conklin Rossant, the arch looks fondly back to Washington's official pre-modern style and, back beyond that, to the grandeur that was Rome.

As commission chairman J. Carter Brown pointed out, it seeks inspiration from the grandiose neo-classical fac,ade of John Russell Pope's National Archives building right across the street, and, in general, from the more sedate neo-classicism of the entire Federal Triangle. "It's good to bring that vocabulary to the north side of the avenue," Brown said. "For a long time it's been as if all the people were dressed for a party on the south side of the avenue, with those that didn't receive their invitations on the other side."

The arch will be the free-standing centerpiece of Market Square, a long-planned urban open space on the north side of the avenue that faces the Archives building between 7th and 9th streets NW. From an urban design point of view an arch in this location--smack in the middle of what now is 8th Street--is a brilliant idea. It frames a remarkable Washington vista, the memorable face-off between the Greek Revival portico of the National Portrait Gallery (the Old Patent Office building designed by William Elliott 140 years ago) and the rich Corinthian fac,ade of Pope's building.

Still, as presented by the architects and the PADC, the arch presents formidable problems. There are three basic philosophical approaches to the challenge of designing such a structure: the modern, as represented in Eero Saarinen's striking gateway arch in St. Louis; the post-modern, as seen in Charles Moore's spiffy new Piazza d'Italia in New Orleans; and the pre-modern, as exemplified by the Conklin Rossant scheme for Market Square.

The latter approach is I think the least interesting, least imaginative of the three possibilities. However, it has been coming for a long time. The PADC itself set the tone when it ruled out those high stone pylons and the marble models of the Capitol and the White House designed by Robert Venturi for the Western Plaza. Venturi's design combined elements of a modern approach (the stark pylons, also intended to frame a vista) and a post-modern one (the humorous scale and readability of the building models). By rejecting these elements while retaining the architect's references to the baroque origins of the plan for this city (the outsized map on the floor of the plaza), the PADC in effect said "No" to a lively, multifaceted approach to contemporary monument design.

When asked about his architectural toryism at the commission meeting, architect William Conklin suggested that, willy-nilly, his arch would be read as a contemporary version of an old theme. While there is some truth in this, it rather cavalierly dismisses the issue of quality. The naval arch could be identified as a contemporary structure simply because it isn't very good.

Let's hope it doesn't end up that way. One of the challenges of the program for the naval arch is that it must do double duty, that is, it must be an arch and it must be a band shell for the Navy Band and others. Bringing a place for music to this rather dead part of the avenue is a terrific idea. Designing an arch as a band shell is a terrific headache for which the architects have not located quite the proper medicine.

Because of this requirement, the interior of their arch is not a true barrel vault--it is wider and higher at the performance side (55 vs. 48 feet wide, about 70 vs. 65 feet high). This will make for a rather odd-looking arch, especially as the architects plan to shelve temporary acoustic panels in its sides, unless, as sometimes happens, the problem provokes the architects to a feat of invention.

The bigger problem of the Conklin Rossant design as it stands is the plainness of its granite and limestone exterior, relieved only by some rather minimal stonework and cornice details, and fluted ionic columns (two pairs facing the avenue, single columns facing north on the performance side). But it is too early to judge.

The well-known triumphal arches, from the Arch of Titus in Rome (celebrating among other things the sacking of Jerusalem) to Napoleon's Arc de Triomphe in Paris, are richly inflected with architectural ornaments and big sculptural ensembles. Although the Navy has not announced its choice of a sculptor, the success of this design depends upon the skill and vision of that person and the degree to which sculptor and architect are able to harmonize their labors--a monumental challenge.

A third major problem is the setting. In the PADC model the arch stands in the foreground of a slightly terraced court where audiences could gather for concerts. This court is to be flanked on either side by massive buildings, each with a two-story shopping arcade at ground level. The design of these buildings and the arcade are absolutely crucial to the success of the whole scheme for Market Square and yet, as of now, they are nothing but ungainly blocks on a model. In fact, the land itself is divided into several parcels with different owners. Whether PADC can coax these owners and their architects to design a proper setting will be a giant test of its mettle.

These are very large ifs. It will take a lot of luck and inspiration, plus heroic determination on the part of the PADC, for us to get, in Carter Brown's words, "an arch that really sings."