1.4. THERMAL MOVEMENT OF STRUCTURAL STEEL (884 Words)

       One of the most difficult things to evaluate throughout the life of a building, and particularly during the construction period, is the amount of horizontal movement, expansion and contraction. It is difficult to design for movement since the designer cannot control some of the parameters. Expansion or contraction requirements for a structure under construction will be determined by the greatest change in temperature that the structure is exposed to prior to being enclosed and conditioned. Thermal movement is a concept that is not unique to exposed structural steel. In fact, it is not unique to steel as a building material. Movement applies to all building materials and must be accounted for in all types of construction. However, for these purposes discussion will be limited to movement of structural steel resulting from changes in temperature.

       For example, it is reasonable for a steel building that is under construction in the Midwest to be erected in summer where the temperature of the steel exposed to the sun can exceed 100° Fahrenheit. The same building may not be enclosed by January, when the night temperatures can dip well below zero. The building would see a temperature change of more than 100° Fahrenheit from summer to winter. 

       The type of temperature differential might not appear to be significant. The integrity of the steel structure would not be affected by the thermal changes. However, the movement and stresses in the steel structure associated with a 100change in temperature could be substantial. The movement and changes in stress of steel are related to the steel's coefficient of linear expansion. The coefficient of linear expansion (or contraction) for any material is defined as the change in length (per unit of length) for a one degree change in temperature. The coefficient of linear expansion for steel is 0.0000065 for each degree Fahrenheit.

       To determine how much a piece of steel will expand or contract throughout a change in temperature, the following equation is used:

Change in steel length = (0.0000065) x (Length of steel) x (Temperature differential)

       If a building with a large rectangular floor plan is exposed to a temperature differential of 60° Fahrenheit, and has expansion joints at every 200 ft in the long direction (see Figure 15), the horizontal movement in that direction will be as follows:

Change in steel length = (0.0000065) x (200 ft) x (60° Fahrenheit)

                                        = 0.08 ft

                                        = 0.94 in.

       It should be noted that this is the total horizontal expansion or contraction that would be expected within that temperature range. If the building were constructed during the coldest temperature of the 60° temperature range, each 200-ft segment between expansion joints would expand approximately 0.94 in. Conversely, if the building were constructed during the warmest temperature season, each 200-ft segment between building expansion joints would contract by approximately 0.94 in.

       Realistically, each expansion joint in this example should be at least one-inch wide if not more. Remember, building construction tolerances must be considered, and a one-inch joint may not be sufficient. The separate sides of the expansion joint should never come in contact with each other even when the building has fully expanded. It should also be noted that the floor, wall, and ceiling finish materials that are selected to cover the expansion joints should be able to accommodate the one inch movement. This would also be true of any mechanical, electrical or plumbing components that span across the expansion joints.

       The previous example is a simplified explanation of building movement. There are, however, other factors that contribute to the "real world" thermal movement of buildings. One of those factors is the fixity of the column bases. If the column bases are "fixed", the thermal movements will be less than with "pinned" base connections. The stress in the members, however, would increase substantially. Other factors, such as whether or not the building is heated and cooled in its designed environment will have an impact on the building's movement.

       An excellent reference on the topic of thermal expansion and contraction is the Federal Construction Council's Technical Report No. 65, Expansion Joints in Buildings. A structural engineer should be consulted before determining expansion joint locations, sizes and spacings.

       Once expansion joint locations and sizes have been determined, accommodations must be made for the movement. Basically, there are two ways to accommodate movement. One way is to provide support members such as columns on both sides of the expansion joint as shown in Figure 16. In essence, the structure on each side of the expansion joint is treated as a separate structure, free to move independently of the other side. The other approach is to make provisions for movement by allowing some of the structure to slide relative to the other while still supported on a common support. This is typically accomplished by creating a seated slide-bearing detail that is supported directly on either a column or a beam as shown in Figure 17. This alternate type of expansion joint is generally used when double columns cannot be accommodated, or where double columns in an exposed position

of the building would be undesirable.

       Regardless of what type expansion/contraction system is used, it cannot be overemphasized that freedom of movement must be incorporated throughout all of the building systems. Again provisions must be made for all components that cross the expansion joint.

(Design with Structural Steel - A Guide for Architects)