Article and photos by Gregory Havel
Before a structure is built, architects and engineers must prepare plans. Before plans can be prepared, information must be obtained from the owner: the size of the proposed building; required ceiling heights; the number of floors; preferred appearance both inside and out; the intended use and location of each room or space; preferred type of heating, ventilating, air conditioning, and temperature control; lighting; location and types of connections for communications and data systems; and budget.
As the architects and engineers prepare the plans, they combine the owner’s requirements with those of national, state, and local building codes and with standard engineering practices. The resulting plans include accessible entrances and other facilities; fire detection, alarm, and suppression systems; fire-rated walls, door, and frame assemblies; exit and emergency lighting; spaces inside walls and above ceilings to conceal pipes, ducts, conduits, and cables; thermal and acoustic insulation; and space for HVAC equipment, plumbing, electrical, communications, and data systems.
These plans include the results of the calculation of the required strength of supporting columns and walls; structural floor assemblies; and roofs and their supports. The calculations are based on the intended use of the rooms and spaces in the building, the furniture and equipment to be provided, the intended number of occupants, and a safety factor. The results of the calculations are split between “live loads” and “dead loads.”
Dead loads are the precisely calculated weight of the building’s structural components (walls, columns, beams, floors, roofs) and permanently-installed equipment (pipes, ducts, conduits, transformers, plumbing fixtures, HVAC equipment), plus a safety factor.
Live loads are the anticipated weight of furniture, merchandise, occupants, the contents of pipes and water heaters, and the forces exerted by wind, rain, snow, and earthquake. These are variables that move around, and may not always be present. Live load calculations are usually based on the minimum requirements stated in applicable building codes which include a safety factor. Some architects and engineers exceed the minimum code requirements in their designs, because they prefer higher safety factors, or because they are anticipating future changes in the use of the building.
The design load figures in the table below are from the plans for an activity building on a school campus in southeastern Wisconsin. They are similar to the loads calculated for similar commercial and apartment buildings in this area. All of the loads are expressed in pounds per square foot (psf).
Note that the greater the live load, the greater the dead load. Greater live loads require stronger structures to support them, and these structures weigh more. Also note that the roof is designed to support about ¼ the live load of corridors, exits, and assembly areas; and about ½ the live load of office space (and resident space in apartment buildings).
When we go on roofs at fires and other incidents, we are often so focused on our assigned task that we assume that the roof will behave much like a floor, even though it was designed only to keep out the weather and to support the incidental weight of maintenance personnel. We often do not recognize that the structure of the roof was designed with less strength and mass than the structure of the floors in the same building; and that it will fail more rapidly when exposed to fire than will the floor. A roof is not a floor!
The photo above shows the second floor deck and roof framing of the building described in the table above. The second floor deck is of hollow-core concrete plank, designed with a dead load of 95 pounds per square foot and a live load of 100 pounds per square foot. The roof framing is of steel bar joists (trusses) supported on reinforced masonry walls, and on I-beams on steel columns. A bundle of the steel roof deck panels is visible above the I-beam at the far right. From the far left to the right, the spaces will be a corridor; washrooms; and a mechanical equipment room, with the HVAC unit (under the blue plastic) set in place with a crane before the bar joists were installed. Simply by noting that the dimensions and mass of the roof supports (bar joists) are smaller than the dimensions and mass of the floor supports (concrete plank), we can infer that the roof assembly was designed for a significantly smaller live load than the floor assembly; and that it is likely to fail more quickly in a fire. The same principle that smaller live loads and more rapid failure in fire are the product of smaller dimensions and smaller mass also applies to structures built of wood trusses and wood I-beams. Again, the roof is not a floor!
|Gregory Havel is a member of the Town of Burlington (WI) Fire Department; retired deputy chief and training officer; and a 30-year veteran of the fire service. He is a Wisconsin-certified fire instructor II and fire officer II, an adjunct instructor in fire service programs at Gateway Technical College, and safety director for Scherrer Construction Co., Inc. He has a bachelor’s degree from St. Norbert College. He has more than 30 years of experience in facilities management and building construction.|