Applies To
	Product(s):	STAAD.Pro
	Version(s):	All
	Environment:	N/A
	Area:	Steel Design
	Subarea:	General Steel Design Solutions
	Original Author:	Bentley Technical Support Group

Can you provide me with some help on how I can include deflection check as one of the criteria in steel design?

Deflection of a beam or a column can be included as one of the criteria during code checking or member selection with most steel design codes in STAAD. The ratio of length to maximum deflection of a beam (L/d ratio) will be calculated by STAAD. STAAD will then check that quantity against the allowable limit which the user specifies under the PARAMETERS option.

What are the design parameters which control deflection check?

1. DFF : This is the value which indicates the allowable limit for L/d ratio. For example, if a user wishes to instruct the program that L/d
cannot be smaller than 900, the DFF value should be specified as 900. The default value for DFF is 0. In other words, if this parameter is not
specified as an input, a deflection check will not be performed.

2. DJ1 and DJ2 : These 2 quantities affect the "L" as well as the "d" in the calculated L/d ratio. They represent node numbers that form the basis for determining L and d.

By default, DJ1 and DJ2 are the start and end nodes of the member for which the design is being performed, and "L" is the length of the member, namely, the distance between DJ1 and DJ2. However, if that member is a component segment of a larger beam, and the user wishes to instruct STAAD that the end nodes of the larger beam are to be used in the evaluation of L/d, then
he/she may input DJ1 and DJ2 as the end nodes of the larger beam. Also, the "d" in L/d is calculated as the maximum local displacement of the member between the points DJ1 and DJ2. The definition of local displacement is available in Section 5.42 of the STAADPro Technical Reference Manual, as well as in Example problem # 13 in the STAADPro Examples Manual.

A pictorial representation of DJ1 and DJ2, as well additional information on these topics is available under the "Notes" section following Table 2.1 in Section 2.8 of the STAADPro Technical Reference Manual.

What are the results one gets from STAAD for the deflection check?

If the steel design parameter called TRACK is set to 2.0, the L/d ratio calculated for the member can be obtained in the STAAD output file. The value is reported against the term "dff". Notice that the expression is in lower-case letters as opposed to the upper-case "DFF" which stands for the allowable L/d.

If "dff" is smaller than "DFF", that means that the displacements exceeds the allowable limit, and that leads to the unity check exceeding 1.0. This is usually a cause for failure, unless the RATIO parameter is set to a value higher than 1.0. If "DFF" divided by "dff" exceeds the value of the parameter RATIO, the member is assumed to have failed the deflection check.

What are the limitations of this check?

Since the "d" in L/d is the local deflection, this approach is not applicable in the case of a member which deflects like a cantilever beam.
That is because, the maximum deflection in a cantilever beam is the absolute quantity at the free end, rather than the local deflection. Check whether STAAD offers a parameter called CAN for the code that you are designing to. If it is available, set CAN to 1 for a cantilever style deflection check.

Since the deflection which is checked is a span deflection and not a node displacement, the check is also not useful if the user wishes to limit story drift on a structure.