C-L3L3 Drift

Drift (lateral deflection) in a steel building is a serviceability issue primarily from the effects of wind. Drift limits are imposed on buildings to minimize damage to cladding and to nonstructural walls and partitions. Lateral frame deflection is evaluated for the building as a whole, where the applicable parameter is the total building drift, defined as the lateral frame deflection at the top of the highest occupied floor divided by the height of the building to that level, $\Delta / H$. For each floor, the applicable parameter is interstory drift, defined as the lateral deflection of a floor relative to the lateral deflection of the floor immediately below, divided by the distance between floors, $\left(\delta_{n}-\delta_{n-1}\right) / h$.

Typical drift limits in common usage vary from $H / 100$ to $H / 600$ for total building drift and $h / 200$ to $h / 600$ for interstory drift, depending on building type and the type of cladding or partition materials used. The most widely used values are $H$ (or $h) / 400$ to $H$ (or $h) / 500$ (ASCE, 1988). These limits generally are sufficient to minimize damage to cladding and nonstructural walls and partitions. Smaller drift limits may be appropriate if the cladding is brittle. AISC Design Guide 3 (West et al., 2003) contains recommendations for higher drift limits that have successfully

been used in low-rise buildings with various cladding types; it also contains recom- mendations for buildings containing cranes. An absolute limit on interstory drift is sometimes imposed by designers in light of evidence that damage to nonstructural partitions, cladding, and glazing may occur if the interstory drift exceeds about 3/8 in. (10 mm), unless special detailing practices are employed to accommodate larger movements (Cooney and King, 1988; Freeman, 1977). Many components can accept deformations that are significantly larger. More specific information on the damage threshold for building materials is available in the literature (Griffis, 1993).

It is important to recognize that frame racking or shear distortion is the real cause of damage to building elements, such as cladding and partitions. Lateral drift only captures the horizontal component of the racking and does not include potential vertical racking, as from differential column shortening in tall buildings, which also contributes to damage. Moreover, some lateral drift may be caused by rigid body rotation of the cladding or partition which by itself does not cause strain and, therefore, damage. A more precise parameter, the drift damage index used to measure the potential damage, has been proposed (Griffis, 1993).

It must be emphasized that a reasonably accurate estimate of building drift is essential to controlling damage. The structural analysis must capture all significant components of potential frame deflection, including flexural deformation of beams and columns, axial deformation of columns and braces, shear deformation of beams and columns, beam-column joint rotation (panel-zone deformation), the effect of member joint size, and the $P-\Delta$ effect (Charney, 1990). For many low-rise steel frames with normal bay widths of 30 to 40 ft (9 to 12 m), use of center-to-center dimensions between columns without consideration of actual beam-to-column joint size and panel-zone effects will usually suffice for checking drift limits. The stiffening effect of nonstructural cladding, walls, and partitions may be taken into account if substantiating information (stress versus strain behavior) regarding their effect is available.

The level of wind load used in drift limit checks varies among designers depending upon the frequency with which the potential damage can be tolerated. Many designers use a 50-year, 20-year, or 10-year mean recurrence interval wind load when checking serviceability limit states (Griffis, 1993; ASCE, 2022).

It is important to recognize that drift control limits by themselves, in wind-sensitive buildings, do not provide for comfort of the occupants under wind load. See Section L5 for additional information regarding perception of motion in wind-sensitive buildings.