Design Checks for Moment Connections

These checks are unique to certain types of moment connections. Moment checks that are standard bolt and material checks, see the Bolt & Material Checks topic. For weld information, see the Weld Checks topic.

Required Flange Force

All moment connections can resist axial, shear, and moment forces. The axial and moment force is converted into a Required Flange Force (Tension and Compression) which is reported at the top of the results Report. This value (sometimes tension, sometimes compression) is then compared to the Available capacity of each limit state in the Unity Check value.

The Required Flange Force is calculated as:

End Plate Moment Checks

These checks are specific only to end plate moment connections. There are several checks for plate failure in an end-plate moment connection. These checks are outlined below. All design procedures and equation references, unless otherwise noted, are taken from AISC Design Guide #4, 2nd Edition. In cases where bolt prying may affect the design, the prying effects are calculated per the procedure in AISC Design Guide #16.

Currently the program can model any type of end plate connections described in the AISC design guides, plus an 8 bolt extended connection not specifically described in the design guides. The yield lines for this "8 Bolt (unstiffened)" connection are taken from Emmet Sumner's PhD thesis ("Unified Design of Extended End- Plate Moment Connections Subject to Cyclic Loading" June 17, 2003) which forms much of the basis for Design Guides 4 and 16. Specifically refer to the MRE 1/2 connection in this paper (section 4.4.4.7 and Figure 4.9) for the end plate yield lines. Similarly refer to the MRE 1/3 connections in Figures 4.10 and 4.11 for the end plate yield lines of the unstiffened and stiffened connections, respectively.

For flush end plate moment connections the g_rvalue is always assumed to be equal to 1.25 to limit the connection rotation at ultimate moment and allow the connection to qualify as a fully restrained moment connection.

Note

• There is a discrepancy between the definitions of h₀, h₁, h₂, in the AISC Design Guides for bolted end plate connections. Design Guide 4 is the more recently published document and reflects a more refined procedure. In Design Guide 4, the h values defined are based on the center-line of the compression flange. This represents the point of zero displacement for the derivation of the yield lines. Therefore, the Design Guide 4 definition is the basis for all bolted end plate yield lines used in RISAConnection.
• Because design is based on the AISC Design Guide, which assumes that half the bolts are in compression, half are in tension, RISAConnection will always check just straight bolt shear (not combined bolt tension and shear). If the user-input loaded results in a large axial tension which puts all bolts into tension, the program results will not change, but a warning will be given to the user in the affected limit states.

Bolt Moment Strength (no prying) 

This check is calculated for the bolts in tension on the End Plate Moment Connection. Expand this section of the design report and RISAConnection will provide you with the exact equation, code reference, listed variables, as well as the code check value and pass or failure notification.

The moment capacity for this check is based on Bolt Tensile Strength from section J3.6 of the AISC 360-10 specification and the Bolt Tension Strength equations (3.7 and 3-8) from the AISC Steel Design Guide 4 (Second Edition). The variables are displayed graphically on page 15 of the Design Guide in Figure 2.7.

Note:

• The 4 bolt connections will automatically consider bolt prying action if applicable.
• When a thin column flange results in bolt prying, it is recommended that stiffeners be provided to prevent prying. If not then the Qmax values calculated will be based on the same formulas given for end plates, but using the column flange properties instead.
• The 8 bolt connections will fail the connection whenever it is determined that bolt prying must be considered. This is because Design Guide 16 does not provide Qmax formulas for the 8 bolt connections.

Verify Bolt Prying Assumption

The Bolt Moment Strength check varies depending on whether there is prying force action or not. Therefore, the program reports this section to show the calculations which determined what Bolt Moment Strength check was used.

This check is covered in the AISC Design Guide #4 "Extended End-Plate Moment Connections", 2nd edition, Copyright 2003 in Section 3.3. We check both the end plate and the column flange for "thick" plate behavior.

F_np - The no-prying bolt tension rupture strength, found in the Bolt Moment Strength check.

F_pl - The end plate bending strength, found in the End Plate Flexural Yielding check.

F_cf - The column flange flexural strength, found in the Column Flexural Yielding check.

For 8 bolt moment connections, if either these checks fail, then the no prying assumption is not valid. Thus, the connection will be said to "fail". IN reality, it may work fine, but the user would have to demonstrate how to manually consider prying for the bolt tension checks.

Beam Flange to End Plate Stiffener Buckling

In order to prevent buckling of the stiffener plate, the stiffener plate thickness is checked per eqn (3.16) of the AISC Design Guide #4, 2nd Edition.

Beam Web to End Plate Stiffener Buckling

For flush end plate connections, no capacity checks are made on any stiffeners specified between the beam web and end plate.

Note:

• For flush end plate stiffeners, the stiffener is assumed to be half the thickness of the beam flange.
• For flush end plate stiffeners between bolt lines, the stiffeners are assumed to be located at the mid-point between the bolt.
• For flush end plate stiffeners located outside the inner tension bolts, the user may control the stiffener location with the Stiffener Pitch dimension.

End Plate Flexural Yielding

The program uses the procedures and tables from the AISC design guides, to determine an end-plate bending strength, and a no-prying bolt tension rupture strength.

If the plate bending strength exceeds 110% of the bolt rupture strength then thick-plate behavior is confirmed and a code check is given based on the other failure modes of the connection. If the plate bending strength does not exceed 110% of the no prying bolt rupture strength, then the program will calculate bolt prying forces and connection capacity per the procedures given in Design Guide 16.

Note:

• For some connections, the program will not give code checks to end plates which experience thin-plate behavior. If this behavior is reported, increase the thickness of the end plate.

End Plate Shear Yielding

The extended portion of the end-plate is checked against shear yielding due to out-of-plane shear (the flange force of the beam). See Eqn 3.12.

End Plate Shear Rupture

The extended portion of the end-plate is checked against shear rupture due to out-of-plane shear (the flange force of the beam). See Eqn 3.13.

Eight Bolt Unstiffened End Plate

This bolt configuration cannot be found in either AISC design guide. However, the yield lines are identical to that shown for the MRE 1/2 configuration shown Design Guide 16. The moment strengths based on the bolts need only be modified by accounting for additional bolts at the top row as shown in the image below.

Asymmetric End Plate Connections

End Plate Moment Connections that are flush on both sides or extended on one side and flush on the other may have a different bolt configuration on the tension side of the connection than on the compression side. This changes the yield line patterns when the "compression" side of the connection goes into tension. This will generally happen when there is moment reversal, but can also occur for connections with have a significant axial tension force.

When this occurs, the output reports only report the limit states for the controlling side. To clearly identify when this situation occurs, the program flags the limit state with text that says "(compression side)" as shown in the image below:

Flush End Plate Connections

RISAConnection always assumes that Flush End Plate moment connections will always be designed as Type 1 or Fully Restrained moment connections. That means that g_r value is always applied to the design of the plate. This is done by increasing the demand force or moment associated with the end plate limit states.

Note:

• The g_rvalue is not used to increase the demand force on the bolts or on the column web. This is based on the examples given in AISC Design Guide 16 and because those limit states are not expected to create excess non-linear rotation that would limit the rigidity of the connection.
• The g_r is used to increase the required strength for column flange yielding. The use for this limit state is a matter of engineering judgment, but is applied in RISAConnection because it is felt that excessive column flange bending would result in a similar type of non-linear rotation as end plate yielding.
• The end plate stiffeners are not designed in a Flush End Moment Plate Connection. Their presence only effects which yield line parameter is used for design per the AISC Design Guide 16.

Axial Force in Moment Connections

When an axial force is added to a moment connection, it is assumed that this axial force is carried entirely by the flanges (Please seeRequired Flange Force). Therefore, the axial force will not affect the shear checks for the moment connection.

Note:

• There are cases where the axial force will actually REDUCE the design checks on a moment connection. This can happen for flange force failure modes that are not valid for both tension and compression. Example: Flange Local Bending (which is valid only for the tension flange) combined with a compressive axial force in the beam, or Web Buckling (which is valid only for the compression flange).

Moment Connections with Column Stiffeners

If the moment connection includes transverse column stiffeners and / or web doubler plates then additional design checks will be performed on these elements. Refer to the Stiffener Checks for Moment Connections topic for more information.

Moment Connections into a Column Web (Weak Axis)

RISAConnection allows you to model both Direct Weld, Flange Plate Moment and Flange Plate Cap Plate Moment connections into the web of a wide flange column.

When you select this orientation, the program will automatically include Flange Plate stiffeners that connect to the beam flanges and the inside column flanges. The Flange Plates are assumed to always be welded to the inside flanges of the column. In the case of a cap plate, the cap plate may be welded to both the inside and outside flanges of the column. You may select a Full Width, Tapered, or Coped end that connects to the beam. The flange forces are then resisted by the flange plate weld strength at the column.

Note:

• Beam to column web connections are not available for seismic moment connections or Canadian (CSA) design codes.

Flange Plate Compression Unbraced Length

RISAConnection considers the full unbraced length of the compression flange in the Flange Plate Compression check. This is measured from the far edge of the weld access hole on the beam to the weld at the column flange.

Cap Plate Stiffener Compression Unbraced Length

RISAConnection considers the unbraced length of the cap plate in the Cap Plate Stiffener Compression check to be the clear distance between column flanges.

Flange Plate with Cap Plate General Assumptions

The cap plate acts as both a flange plate connected to the top of the beam flange and a transverse stiffener that is connected to the top of the column. The design force used for the cap plate is the beam flange force generated by the moment within the beam. The force developed within the top beam flange is transferred directly into the column through welds at the top of the column. Here are some general assumptions used by RISAConnection.

• The cap plate and flange plate will be the same length along the beam
• The flange force in the flange plate and cap plate are always assumed to be equal despite differences in the cap plate thickness and lower flange plate thickness (i.e. having different stiffnesses)
• Column capacity is not consdered in the determination of an unbalanced flange force for the cap plate. In other words, the cap plate will always be conservatively designed using the full flange force.
• HSS column checks are only performed at the lower flange plate and not at the top cap plate.
• Welds
  • Weld lengths along a wide flange column web can not be adjusted
  • Weld lengths along wide flange are dictated by the cap plate width and can not be adjusted independently
  • Weld lengths will always be the full length and width for HSS columns
• Seismic Detailing is not available for Flange Plate Moment Column Cap Plate

Continuous Beam over Column Moment Connection

Guidance from the AISC Design Guide 24 is used for the connection checks for the continuous beam over column connection. The connection can support axial load onto the column (from the beam), strong axis bending moment (bending perpendicular to the beam’s length), and shear forces in either direction. Weak axis bending moment (bending parallel to the beam’s length) is not supported. Additional guidance from AISC Design Guide 4 and 16 are used when Design Guide 24 does not provide sufficient information.

Due to limited guidance for newer AISC codes from Design Guide 24; HSS column wall local checks (yielding and crippling) are completed using more conservative AISC equations. For AISC 15th edition, the yielding check is calculated per AISC J10-3 and the crippling check is calculated per AISC J10-5a.