Portable Classrooms


Schools around the country have been faced with growing enrollment requiring the construction of thousands of new classrooms in the past few years. The vast majority of this construction has been done utilizing traditional site built methods.

In recent years, the techniques of prefabrication, long a part of the housing industry, have been increasingly applied to the construction of classroom buildings.

What is Modular Construction?
The terms "modular" or "portable" refer to a wide range of methods for pre-fabricating off-site (in a factory setting) whole buildings or parts of buildings, shipping them to the site on trucks, and assembling the pieces onto site-built foundations.

The difference between the methods is chiefly defined by the degree of pre-fabrication. This varies from "portable classrooms" where virtually 90% of the value of the finished building is built off-site to component pre-fabrication, which may involve a relatively small percentage of off-site construction. This is sometimes referred to as "permanent modular" construction.

There are countless examples of portable classrooms used throughout the country. These classrooms are generally of wood frame construction, come completely finished inside and out, and are limited in size to the constraints of the trucks and the roads over which they are carried. They are meant to be relocatable, even though some states require that they be placed on permanent foundations and have permanent utility hook-ups and fire resistant access (stairs and ramps).

The examples of permanent modular construction that can be found locally generally fall into two categories:   panelized structural components and prefabricated building modules.

Panelized structural components involve the prefabrication of light gauge steel framing panels that are erected on a site built foundation and slab. The size of the panels is limited only by the height of underpasses on the delivery route and includes the framing for the exterior and interior walls, floors and roofs. In this system, buildings are limited to two stories due to the structural and fire resistance limitations of light gauge steel framing. In addition clear spans are limited to about 32 feet, which would preclude gymnasiums and probably cafeterias. Otherwise, there is good flexibility in configuration. The heating, ventilating, air conditioning, electrical and plumbing systems, as well as all interior finishes, are installed on site. The exterior cladding, ranging from synthetic stucco to brick, the roofing membrane, and the windows and doors are also installed on site. A conservative estimate would be that 80% of the value of the project is built on site.

Prefabricated building modules involve the prefabrication of whole building modules including floors walls and ceilings that are shipped to the site, set on foundations and welded together to form finished spaces. This type of construction involves a combination of structural and lightweight steel framing and concrete floors combined in modules that can be stacked to form multi-story structures. The mechanical, electrical and plumbing systems are pre-installed in the building modules with only field connections between modules required on site. The interior finishes, along with the exterior cladding and windows, are also pre-installed in each module. The size of the module is limited by local road conditions over which the modules must be trucked. Each building project is unique and so must be extensively pre-engineered to insure proper coordination at the final assembly on site.

Although it is possible to bid the work in a conventional manner, this type of construction is generally used in conjunction with a design-build methodology that places the ultimate design responsibility on the contractor

Criteria for Evaluating Alternative Construction Methods

The most important criteria for evaluating which of these alternative methods of construction make the most sense for a specific school building project are as follows:

Configuration/planning flexibility: Does the construction method meet the design and planning requirements by allowing the desired number of stories and arrangement of rooms and corridors? Can the construction method incorporate long-span spaces such as a gym or cafeteria?

Construction quality/durability: How well will the finished building stand up to the wear and tear of serving as a heavy-use school facility? What is its expected useful life? What will be the annual operating costs of keeping it in working order and maintaining its appearance?

Construction time: How long will the building take to construct? How much disruption will construction cause to the ongoing school operation? Will a given construction method reduce the amount of disruption, allow substantial construction to take place during a vacation period when the school is not operating, and avoid potential weather-related delays?

• Construction cost: What is the relative cost of each method of construction? Does a given construction method lower costs sufficiently to offset possible disadvantages associated with it?
• Flexibility: Within the parameters of a given construction method, can the architect's design be translated into an attractive finished building that meets community expectations?