Seismic Detailing - Design Rules

Seismic Design Rules can be applied to Column, Beam and VBrace members in the model. The rules invoke various design or code check requirements according to AISC seismic design provisions (AISC 341, AISC 358). The program uses the 2005, 2010, or 2016 versions of these codes, depending on the HR steel design code settings in the Model Settings.

The design provisions primarily apply only to Hot Rolled steel members. However, the Column Overstrength design option applies to all members to which that seismic design rule has been applied. The reason for this is so that members (collectors or such) which require design to the overstrength provisions per the requirements of ASCE-7 can be automatically designed to the higher force requirements of the Overstrength load combinations.

The default entry for a member's seismic design rule is None which means that no special seismic detailing provisions will apply to the code checking provisions for that member.

General Frames and Columns

Seismic Design Rules: Hot Rolled Frame - General Columns

The first portion of the Seismic Design Rules under the Hot Rolled Frame tab applies to the steel frame in general and Hot Rolled columns.

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Seismic Design Rules: Hot Rolled Frame - General / Column Ductility

Column Header	Description
Label	The label is a user defined text string which is used as a unique identifier for each of the seismic design rules defined for the structure. The program comes pre-loaded with number of generic seismic design rules based on AISC 341 seismic detailing specification. Click on the ellipsis in the Label column to open the Seismic Design Rule Editor window. This window lets you further define the frame type, member ductility, and member overstrength requirements, etc. You can choose either to use this editor window or the spreadsheet, to edit a seismic design rule. Click on image to enlarge it
Frame Type	Frame Type defines the frame type (e.g. SMF, SCBF) for the frame. The program is pre-loaded with several typical seismic-force-resisting-systems based on AISC 341 seismic detailing specification. The program uses this entry to determine which code sections of seismic detailing requirements should be applied to members with the current seismic design rule.
Column Ductility	Column Ductility defines the ductility requirements for the columns. High Ductility:Refers to a member which requires Seismically Compact sections for members such as a Special Concentrically Braced Frame (SCBF) or a Special Moment Frame (SMF). Moderate Ductility: Refers to a member which require Compact sections for frame members, but which does not require the special Seismic Compactness defined in AISC 341. One example would be an Intermediate Moment Frame (IMF). Minimal Ductility: Refers to a member which does not have specific compactness requirements beyond the normal AISC specification. This can even include members with slender elements. One example would be an Ordinary Moment Frame (OMF).
Note: Ordinary Concentrically Braced Frames (OCBF) require Moderate or High ductility out of the brace members (depending on the version of the code), but allow for Minimal ductility out of the other members. The program will automatically account for this brace ductility requirement. Therefore, the Frame Ductility may still be entered as Minimal. When checking the seismic compactness of members, the axial load used to calculate Ca is based on the worst case axial compression from the normal load combinations. It will not consider any axial force that occurs from an "overstrength" load combination. The Frame Ductility setting is used by the program when it is checking some of the miscellaneous beam-column moment connection requirements per AISC 358. For example: if the beam to column connection is specified by the user as a Reduced Beam Section (RBS) then the span to depth ratio of the beam must be greater that 7.0 to be considered a highly ductile frame, and greater than 5.0 to be considered a moderately ductile frame.
Column Overstrength	In seismic design some members may be required to be designed to an elevated / overstrength load. In RISA, this is done by creating load combinations with the Omega overstrength factors applied to the earthquake loads. If the column members are required to be designed to these load combinations, then this check box will be checked.
Note: In many cases, it's only the axial effect of the overstrength loads that needs to be considered in the column overstrength code checks. In these cases, it could be overly conservative to consider the moment with these elevated axial forces. A future revision to the program will add an Axial Only option to this input field which would allow the program to ignore the effects of moment when the amplified axial force is taken into account. Generally speaking the 2005 version of AISC 341 requires that moment frame columns be designed for the overstrength loads whenever the axial force for the regular load combinations exceed 40% of the column's axial capacity. However, the 2010 and newer versions of the same specification always require that these columns be designed to these overstrength loads. Therefore, this entry cannot be modified when the 2010 or newer code is selected. The program does not consider any of the limitations related to the "sum of shears which can be transmitted to the column" or the "sum of the expected strength of the braces". If a Seismic Design Rule is applied to a column member which is not hot rolled steel (such as a wood drag strut or collector), that member will ignore the design rules except for the Overstrength required flag. If this is selected, then the non-steel member's capacity will be checked against the forces derived from the overstrength seismic load combinations. Refer to the LC Generator and the Set BLC Entry sub-topics in the Load Combinations section for more information regarding the creation of overstrength load combinations.

Beams

Seismic Design Rules - Hot Rolled Frame - Beams

This portion of the spreadsheet contains information pertinent mostly for the design of hot rolled beams that are part of moment frames. The only exception to this is the Beam Overstrength option which applies to any beam member which is assigned the seismic design rule.

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Seismic Design Rules: Hot Rolled Frame - Beam Ductility

Column Header	Description
Beam Ductility	Beam Ductility defines the ductility requirements for the beams. The definitions of High Ductility, Moderate Ductility, Minimal Ductility are described under the Column Ductility section above.
Connection	Connection designates which of the pre-qualified moment connections defined in AISC 358 or its supplement are being used for the beam to column moment connection. The options are: Bolted Flange Plate (BFP) as described in Chapter 7 of AISC 358. Reduced Beam Section (RBS) as described in Chapter 5 of AISC 358. Bolted Unstiffened Extended End Plate (BUEEP) and Bolted Stiffened Extended End Plate (BSEEP) as described in Chapter 6 of AISC 358. Welded Unreinforced Flange - Welded Web (WUF-W) as described in Chapter 8 of AISC 358. Kaiser Bolted Bracket (KBB) as described in Chapter 9 of AISC 358 Other/None: There are times where AISC 341 and AISC 358 have conflicting provisions. This includes the definition of the probable maximum moment at the hinge and the definition of the Strong Column / Weak Beam Ratio. When one of the pre-qualified connections is selected then the program will enforce the AISC 358 version of the provisions. When the Other connection is selected then, the AISC 341 version of these provisions will be enforced instead. This "connection option" should normally be used when the beam will not have moment connections (i.e. it is part of a braced frame). Note: The moment connection setting is used (in combination with the Frame Ductility setting on the column tab), to check some of the miscellaneous beam-column moment connection requirements per AISC 358. For example: if the beam to column connection is specified by the user as an Reduced Beam Section (RBS) then the column depth must be limited to a maximum of W36, and the beam weight cannot exceed 300 lbs / ft.
Beam Overstrength	In seismic design some members may be required to be designed to an elevated / overstrength load. In RISA, this is done by creating load combinations with the Omega overstrength factors applied to the earthquake loads. If the beam members are required to be designed to these load combinations, then this box would be checked.
Note: Beams in a braced frame may act as some form of a collector or drag strut. As such, the ASCE-7 seismic provisions would require that they be designed for the overstrength load combinations. If a Seismic Design Rule is applied to a beam member which is not hot rolled steel (such as a wood drag strut), that member will ignore the design rules except for the Overstrength required flag. If this is selected, then the non-steel member's capacity will be checked against the forces derived from the overstrength seismic load combinations. If only an overstrength load combination was run then you will get no results for members that are not required to be designed to overstrength loading. Refer to the LC Generator and the Set BLC Entry sub-topics in the Load Combinations section for more information regarding the creation of overstrength load combinations.
Z Factor	This factor is used to define the reduction in plastic hinge moment expected for Reduced Beam Sections. Enter in the ratio between the plastic section modulus for the reduced beam section and the unreduced beam. For RBS connections this value will vary greatly, but will always be less than 1.0. The program will not allow a value of less than 0.1 to be entered in by the user. If this value is left blank, then no reduction in moment is considered. This factor will be used to determine the probable design strength and the strong column / weak beam moment ratio for the connection. It does NOT currently reduce the stiffness of the beam used in the analysis. Note: AISC 358 has some restrictions on the length and depth of cut that is allowed for the RBS section. RISA does not make any attempt to enforce these restrictions.
Hinge Location	This entry defines the location of the assumed plastic hinge (in inches) from the face of the column. This is used to determine the design moment at the face of the column as well as the strong column / weak beam ratio. For RBS connections, this entry should be the distance from the face of column to the center of the reduced beam section. For stiffened connections (such as BSEEP, or BFP), this will usually be the distance from the face of column to the end of the stiffener, haunch, or flange plate. For unstiffened end plate connections, this entry will usually be the lesser of 50% of the beam depth or 3 times the beam flange width. For WUF-W connections, this entry will normally be set to 0.0.

Braces

Seismic Design Rules - Hot Rolled Frame - Braces

This portion of the spreadsheet contains information pertinent to the design of hot rolled braces. The only exception to this is the Brace Overstrength option which applies to any VBrace member which is assigned the seismic design rule.

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Seismic Design Rules: Hot Rolled Frame - Brace Ductility

Column Header	Description
Brace Ductility	Brace Ductility defines the ductility requirements for the braces. The definitions of High Ductility, Moderate Ductility, Minimal Ductility are described under the Column Ductility section above.
Brace Overstrength	In seismic design some members may be required to be designed to an elevated / overstrength load. In RISA this is done by creating load combinations with the (Ω₀) overstrength factors applied to the earthquake loads. If the brace members are required to be designed to these load combinations, then this box would be checked.
Note: If a Seismic Design Rule is applied to a brace member which is not hot rolled steel, that member will ignore the design rules except for the Overstrength required flag. If this is selected, then the non-steel member's capacity will be checked against the forces derived from the overstrength seismic load combinations. Refer to the LC Generator and the Set BLC Entry sub-topics in the Load Combinations section for more information regarding the creation of overstrength load combinations.
Max KL/r	In seismic design some members may have the following restriction on the maximum slenderness (KL/r) value that they are allowed to have. Examples would be AISC 341-05 braces that are part of a Special Concentrically Braced Frame (SCBF). This would also include AISC 341-05 and AISC 341-10 K, V, or inverted V braces in an Ordinary Concentrically Braced Frame (OCBF).

Limitations

Seismic Detailing Limitations

Limitation	Description
Panel Zone Capacity	Currently, the program uses the elastic shear capacity equations (AISC 360-2010, eqns J10-9 and J10-10) for its code check of the panel zone. When plastic panel zone deformation is considered (along with shear capacity of the flanges) less conservative equations ( J10-11 and J10-12) may be more appropriate.
V and Inverted V Braced Frames & Gravity Loads	The requirement that the beam be analyzed as though the brace carried no dead or live load cannot be directly met in the RISA-3D analysis. However, this may be accomplished by integrating the model with RISAFloor, which automatically analyzes the beam for gravity loads as though the brace were not present.
Unbalanced Beam Force in Braced Frames	The effect of unbalanced brace forces is calculated only for beams in V or an inverted V frame configuration. This force is reported in the Seismic Detailing portion of the beam's detail report. However, this force is NOT used in the code checking of the beam UNLESS a capacity-limited (CL) load combination is used. For capacity-limited LCs (e.g. LC with ELX-CL), the program will apply brace expected strengths as seismic loads for columns and beams design. Under these load combinations, the beam unbalanced forces will be automatically considered in beam design by applying the capacity-limited forces.
Expected Strength of Braces	The effect of expected strength of braces is considered through the capacity-limited load combinations (e.g. ELX-CL, ELZ-CL) for columns and beams in braced frames such as SCBF and BRBF. Users need to select the supported design code (AISC 2010 or 2016), assign seismic design rules with appropriate frame type (e.g. SCBF or BRBF), and generate capacity-limited load combinations to perform capacity-limited design for columns and beams in braced frames.