Roof Structure: Supporting Mechanical Loads
Originally published in Stanford White’s technical bulletin, Technology In Action.
A project that involves replacement of mechanical equipment in an aging building can often require creative structural design solutions. Since mechanical heating and air conditioning systems tend to lose their serviceability before structural components, owners of aging buildings are often faced with the decision of whether to repair or replace mechanical systems before they have to make the decision of whether to rebuild or relocate. Placement of new mechanical equipment in or on an existing building may require substantial structural reinforcement in order to properly support that equipment, work that could add significant cost to the owner’s construction budget. The extent of required structural work can be driven by the size, weight and proposed location of the new equipment, as well as the integrity and load carrying capacity of existing structural components upon which new equipment loads will be imposed.
One of the most common locations for placement of mechanical equipment is on the roof. Whenever new equipment is placed on an existing roof, it is important to distribute the concentrated equipment loads over as large an area as possible so as to reduce the magnitude of new loads being imposed on individual roof framing members. The most ideal places to locate such loads is at or near the bearing points of roof joists and/or beams, where shear stresses normally govern design rather than bending stresses or deflection criteria, and directly over building columns since columns are the most likely structural component of a building to have been designed with “reserve” load capacity.
Regardless of how or where new equipment loads are distributed, some roof framing systems can more readily support additional loads than others. The type of roof framing system of an existing building can usually be determined by cursory field observations and/or by examination of existing construction documents. The acquisition of existing construction documents, however, is not always possible, especially for very old buildings. Without the benefit of construction documents, extensive field work by the structural engineer may be required, services that would need to be accounted for in the owner’s construction budget.
Following is a list of some of the more common types of roof framing systems encountered in older buildings along with a brief discussion of how likely they are to require structural reinforcement and how such reinforcement can be accomplished.
Open Web Steel Joists
An open web steel joist is one of the most economical structural components available for the construction of floor and roof framing systems. It is typically designed to support only the minimum loads required by the Building Code, which leaves little or no “reserve” capacity for new loads. The application of concentrated loads as low as 300 to 500 pounds can require special structural attention. And, because an open web steel joist is designed as a truss, concentrated loads are to applied only at its top or bottom chord panel points. These features of an open web steel joist often require it to be reinforced before it can support additional loads.
A common way to reinforce an open web steel joist is to weld a structural steel member along its top and/or bottom chord. The required size and length of the reinforcing member is dependent upon the magnitude and location of the imposed concentrated load. Additional reinforcement can be accomplished by welding angles between top and bottom chords.
Cold Formed Steel Purlins
Cold formed steel purlins are typically associated with engineered metal building systems, but they can occasionally be found in the roof framing system of a conventionally constructed building. Like an open web steel joist, a cold formed steel purlin is a very efficient and economical roof framing component and is usually designed to support only the minimum loads required by the Building Code.
The design and fabrication features of a cold formed steel purlin make it one of the most likely of all roof framing components to require reinforcement in order to support additional concentrated loads. Because the purlin is fabricated with light gauge steel, its reinforcement cannot always be accomplished by welding sections of thicker structural steel elements to it. Many times intermediate framing members must be added to the existing roof framing system to support new equipment loads. In some situations the new framing can be placed in the plane of the existing framing while in others the new framing must be placed above the roof covering system. In these situations the new framing must typically span between new stub columns erected on top of existing columns, which adds to the overall construction cost of the support system.
Prestressed, Precast Concrete Tees
Prestressed, precast concrete tees are typically found in buildings with large open areas, such as auditoriums, gymnasiums and natatoriums. These structural elements are usually incapable of supporting additional concentrated loads unless those loads were specifically accounted for in their original design. Reinforcement of concrete tees is made difficult by the nature of their fabrication. Because the prestressing tendons are placed in the stems of the tees, attachment of steel reinforcing to the stems can be a dangerous proposition unless the precise location of the tendons is determined. Rupturing a tendon in a prestressed concrete tee can have a significant detrimental effect on its structural integrity. As a general rule of thumb, placement of new mechanical equipment on an existing roof framed with prestressed, precast concrete tees should be avoided unless the new equipment is of the same size and weight and is placed in the same location as that which it replaces.
Cast-in-Place Concrete Joist/Slab System
Cast-in-place concrete joist/slab systems were commonly used for the floor and roof framing of institutional type buildings constructed in the mid-twentieth century. If properly constructed and maintained, these systems can often be counted on to support additional concentrated loads with little or no reinforcement. One of the keys to successfully placing new mechanical equipment on a concrete joist/slab system is to limit the size of required penetrations through it. While the slab can usually be cut without adversely affecting the structural integrity of the system, the joists must remain intact. If a joist has to be cut, then its cut end must be supported. This is typically accomplished by erecting a structural steel header to distribute its loads to adjacent joists. These joists, in turn, typically have to be reinforced to support the additional load imposed upon them. Unlike the prestressed concrete tees, reinforcement of these joists can usually be accomplished by attaching sections of structural steel shapes to their stems.
Wood Framed Systems
Unless framed with heavy timbers or glue laminated lumber, placement of new mechanical equipment on a wood framed roof will almost always require reinforcement of its framing members. This is particularly true for manufactured wooden trusses, which are usually designed for the lightest loads allowable by the Building Code. However, of all the various roof framing systems described so far, wood framed systems may be the easiest to reinforce. Reinforcement of these systems is typically accomplished by “sistering” pieces of dressed lumber along one or both sides of joists, rafters and/or beams. Manufactured trusses may require top and bottom chord reinforcement as well as reinforcement of diagonal web members, which can be accomplished in the same manner as for joists and rafters.
Once an existing roof framing system has been identified and its reinforcement requirements determined, there are other important factors that must be considered when new mechanical equipment is to be placed on an existing roof, including the following:
1. The type of roof covering system (built-up, rigid insulation with single ply membrane, standing seam, asphalt shingles, to name a few), and the procedures required to adequately repair that system and flash around penetrations;
2. Accessibility to roof framing members from above and below and the means and methods required to reinforce them;
3. Day-to-day operations conducted within the building, the effects of roof construction on those operations and the means and methods required to protect building contents and occupants from falling debris or rainwater intrusion;
4. Screening requirements for new rooftop equipment;
5. Anchorage of equipment in high wind or seismic zones.
If it is determined that an existing roof framing system cannot support new mechanical equipment without structural reinforcement, the equipment can always be placed on “terra firma”, but then, how creative a solution is that?