What are the parameters for Deflection Check in STAAD.Pro?

The design parameters are described in the STAAD.Pro Technical Reference Manual chapter 2.4.

The DFF parameter is used to assign the allowable ratio of span/deflection. The default value for DFF is zero, which implies that no deflection check is desired. In other words, if this parameter is not specified as an input, a deflection check will not be performed. The trigger to invoke a deflection check on any member is to assign that member a non-zero DFF parameter.

The DJ1 and DJ2 parameters are used to identify the nodes at the extreme ends of the member to be evaluated for deflection. This is used to define the span length to be considered when evaluating the deflection of members, such as girders, that consist of multiple beam members connected by intermediate nodes. Identifying the ends of the member also allows the program to net out any displacement of the member ends. In this way, STAAD.Pro provides a true evaluation of the member deflection.

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 5.44 of the STAAD.Pro Technical Reference Manual, as well as in Example problem # 13 in the STAAD.Pro 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 of the STAADPro Technical Reference Manual. If you use the design parameter TRACK 2.0, you will see a term called "dff" in the STAAD output file. This term stands for the actual length to deflection ratio computed by STAAD. If "dff" is smaller than "DFF", it means the member has violated the safety requirement for deflection, and will be treated as having failed.

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. 
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. 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.