In architectural school, teachers of design ask students to focus particular attention on the bottom story or two of buildings and their integration with the pedestrian environments around them. How can we humanize and animate this special realm where buildings meet the ground to form the streetscape?

When you walk by or approach and enter urban buildings, you become increasingly aware of their constituent materials, textures, details, entries and interior content. Only from the sidewalk vantage point is a building's complexion intimately revealed, warts and all.

A building edging the sidewalk engages your interest and invites interaction as you draw near if its base is porous, not opaque. Buildings that present blank, solid, impenetrable walls to pedestrian passers-by do little to animate urban streetscapes. The District has many such inscrutable structures, the FBI Building on Pennsylvania Avenue being among the most illustrious.

Architects frequently connect buildings to the pedestrian realm with colonnaded arcades, presumed to create porosity and openness. Downtown and suburban Washington are full of office buildings whose bottom one or two stories are recessed behind the dominant exterior wall plane at the building setback line. Leaving the structural columns in place creates the colonnade and covered sidewalk behind it. This design tactic has become so conventional and cliched that we almost take it for granted, yet it can lead to questionable results.

Arcades may further widen sidewalks already sufficiently wide. While this allows space for street vendors, it pushes storefronts and shop display windows even farther from the principal pathways of pedestrians and from the street.

On the north sides of buildings, deep arcades never receive sun and remain eternally in shade. Excessively high arcades may offer minimal protection from the descending elements or the summer sun. At night, recessed storefronts share less light with the sidewalk and street.

For centuries, continuous colonnaded arcades have encircled free-standing buildings or wrapped around courtyards within buildings. They have enclosed, either partially or completely, plazas, squares and gardens framed by several buildings. Visualize atria and courtyards within ancient residences, or the great piazzas of St. Peter's in Rome and San Marco's in Venice.

Colonnaded arcades provided ways to walk from one place to another. They were connective as well as decorative. Such connecting links not only made sheltered paths of travel between origin and destination, but they also unifed otherwise separated structures, creating visual, spatial and symbolic ties.

Today, however, if you look along any of the principal streets in the downtown business district, you see series of disjointed, stop-and-start arcades at the bases of one infill building after another. Many lead nowhere . . . except to connect the party wall on the left to the one on the right when abutted by buildings without arcades.

Many colonnaded arcades are not spaces where you would choose to promenade. Pedestrians normally stick to the open sidewalk well outside of arcades and often at a considerable distance from shop windows and doors. Even during inclement weather, the stop-and-start nature of arcades within a block may force you to weave intermittently in and out of facades and in and out of the rain.

Arcade heights and depths vary randomly. In some buildings, the arcade depth is very much less than its height, perhaps only a few feet, barely enough for one or two people to walk between column and wall. In these cases, the arcade seems vestigial and without purpose. Why have it at all?

Many downtown buildings, both old and new, front the sidewalk successfully without arcades. Look at the office buildings at 15th and K NW and 1990 K St. One Thomas Circle and 1400 K St. NW have ground-floor shops that come to the building line and sidewalk. Only at their entrances do their walls step or curve back behind the column line to make recessed loggias.

Washington's amply wide sidewalks permit variety in the treatment of main entrances to buildings and subordinate entrances to stores, banks and restaurants. Canopies, awnings, or other porch-like elements can reach out into and over the public sidewalk, signaling and celebrating the place and act of moving from outside to inside, from public to private domains. These gestures seem particularly appropriate because we often view buildings from oblique angles rather than straight on.

An urban building's entrance need not be gigantic or overwhelming, but it should be visible, well-marked and inviting. Entrance loggias or canopies extending a symbolic hand of welcome to the street have an interior presence as well. An entrance is also an exit, a two-way connector between inside and outside, the portal to the street when viewed from within.

The recessed arcades lining the fronts of so many buildings don't always serve well as porches or loggias to connect inside to outside. Most pedestrians approach such buildings by walking along the principal sidewalk and then veering in toward the entry doors.

Washington, with its wide sidewalks and not-too-high buildings, easily could accommodate arcades similar to those found in Paris, Rome and other cities around the world. Many of their streets are lined by arcades that extend over the public sidewalk to within a few feet of the curb. Just enough space remains between curb and column line for open car doors, parking meters, utility poles, street trees and facade maintenance.

Arcades may be only one story high without occupied building space above. The building proper stays behind the property line. Some arcades run continuously along streets from corner to corner. They also turn corners or penetrate and march into the interior of blocks where courtyards, atria or shopping galleries may be situated.

Arcade roofs can be solid and opaque, or they can have varying degrees of transparency and translucency through the use of finely scaled trellises, glass skylights or domes. In some cases, flat roofs become outdoor terraces or roof gardens for occupants of second or third floors, depending on the height of the arcade below and the activities within the building.

Properly designed arcades can accommodate outdoor eating and sidewalk cafes, passengers waiting for buses or taxis, street vendors, commercial signs, lighting, visible and legible street addresses and vegetation, as well as pedestrians. At the same time, building owners and commercial tenants retain use of valuable square footage at the ground floor level abutting the principal path of movement at the building setback line.

To design and build such arcades in Washington would require modified zoning regulations, coupled with appropriate design guidelines. These would ensure that each building in a zone designated for public sidewalk arcades provided its share of the street arcade in a way compatible with (though not necessarily identical to) adjacent buildings and arcades.,

Even now, sidewalks are appropriated and used for some of these purposes, but without the sheltering and linking arcades. Along Capital Hill's commercial streets, Connecticut and Pennsylvania avenues or 19th Street, you readily can observe and participate in these activities. Sometimes, awnings or umbrellas aid space-making and signal points of access and festivity.

Active sidewalk and street life is one of the wonderful amenities that cities offer. Urban architecture, in addition to looking good, must enhance this life and be part of it.

NEXT: Squares and plazas