* This post was written by Jeff Feaster, P.E., Vice President of Engineering at Whirlwind.
Lateral deflection and steel buildings
Lateral deflection, also known as sidesway, is the movement of a building structure horizontally resulting from loads such as wind load, seismic load, cranes, etc. and even small amounts of secondary movement from vertical loads. A building can be structurally sound from a stress standpoint and still allow for varying levels of acceptable movement, based on the building’s function.
The amount of sidesway limitation can have a significant effect on the cost of building framing. An understanding of this behavior of structures can help an owner to match his function with serviceability to avoid paying too much for the building structure.
Using sidesway ratio to set building movement limitations
A typical standalone metal building with metal wall panels can be allowed to move laterally without concern the movement will cause any negative effects. For example, a building with a 16’ eave height and an allowable H/90 sidesway requirement will move 2.1”, as calculated by the computer, under full design load. This amount of movement would be of no concern. But, 2” may be judged to be too much movement for a building with extensive glass, full height CMU, or other adjacent structures.
Many specifiers have adopted the sidesway ratio as their means of setting building movement limitations rather than identifying a specific amount. For instance, if the specifier were to judge that the building could move laterally 1” at the eave with no deleterious effect on any adjacent building material, that amount could be worked backward into an allowable ratio. A building of 16’ at the eave and an allowable 1” of sidesway would therefore have an allowable of 1” x 16 X 12 or 192. By this reasoning, an allowable sideway ratio of H/200 would be acceptable. However, if the building was 24’, that same 1” of movement would be attained limiting sidesway to H/300.
For adjacent structures below the eave line, a linear relationship is credible. For a building of 16’ eave height and 1” sidesway at eave, the column would move approximately ½” at the 8’ height – a common dimension for many partial height walls.
Bare frame deflection
There is another trait worth noting here. Computer analysis yields a sidesway amount known as “bare frame” deflection. The computer performs an analysis applying a horizontal load at one eave and reflecting the overall stiffness of the frame, delivers a resulting amount of movement. However, the computer model gives no consideration to the actual stiffness of a building system, taken as a whole. In other words, with each additional component (girts, purlins, sheeting, etc.) attaching to the frame, the frame is stiffened further. “Bare frame” deflection cannot physically occur in the field. Therefore, the calculated sidesway amount is very conservative and the actual amount experienced in the field would more likely be half the calculated, or even less.
This information is to provide more guidance to specifiers regarding this topic. Many specs are written with H/360 or even H/600 which can result in an overly conservative structure. In the interest of owners’ project costs, we feel this is an important topic to make them aware of.