Can anyone please tell me where can i find the dff value while designing a steel structure using AISC-ASD Code. I tried to find it in code book but i am confused whether to use 360 or 240. But by default staad takes 300. It will be very thankful if someone can help me with this issue.
Thanks.
Please refer to say, AISC 360-05 codebook chapter L3. DEFLECTIONS, which tells:
"Excessive vertical deflections and misalignment arise primarily from three sources:
(1) gravity loads, such as dead, live and snow loads; (2) effects of temperature,
creep and differential settlement; and (3) construction tolerances and errors. Such
deformations may be visually objectionable; cause separation, cracking or leakage
of exterior cladding, doors, windows and seals; and cause damage to interior components
and finishes. Appropriate limiting values of deformations depend on the
type of structure, detailing and intended use (Galambos and Ellingwood, 1986).
Historically, common deflection limits for horizontal members have been 1/360
of the span for floors subjected to reduced live load and 1/240 of the span for
roof members. Deflections of about 1/300 of the span (for cantilevers, 1/150 of the
length) are visible and may lead to general architectural damage or cladding leakage.
Deflections greater than 1/200 of the span may impair operation of moveable
components such as doors, windows and sliding partitions.
Deflection limits depend very much on the function of the structure and the nature
of the supported construction. Traditional limits expressed as a fraction of the span
length should not be extrapolated beyond experience. For example, the traditional
limit of 1/360 of the span worked well for controlling cracks in plaster ceilings
with spans common in the first half of the twentieth century. Many structures with
more flexibility have performed satisfactorily with the now common, and more
forgiving, ceiling systems. On the other hand, with the advent of longer structural spans,
serviceability problems have been observed with flexible grid ceilings where
actual deflections were far less than 1/360 of the span, because the distance between
partitions or other elements that may interfere with ceiling deflection are far less
than the span of the structural member. Proper control of deflections is a complex
subject requiring careful application of professional judgment. West, Fisher, and
Griffis (2003) provide an extensive discussion of the issues."
Thanks Andre i'll refer what you said. :)
Answer Verified By: Rajive Vijay