E4Special truss moment frames (STMF)

E4.3a Special Segment

The required vertical shear strength of the special segment shall be calculated for the applicable load combinations in the applicable building code.

E4.3b Nonspecial Segment

The required strength of nonspecial segment members and connections, including column members, shall be determined using the capacity-limited horizontal seismic load effect. The capacity-limited horizontal seismic load effect, $E_{c l}$, shall be taken as the lateral forces necessary to develop the expected vertical shear strength of the special segment acting at mid-length and defined in Section E4.5c. Second-order effects at the design earthquake displacement shall be included.

E4.4a Special Segment

Each horizontal truss that is part of the SFRS shall have a special segment that is located between the quarter points of the span of the truss. The length of the special segment shall be between 0.1 and 0.5 times the truss span length. The length-to-depth ratio of any panel in the special segment shall neither exceed 1.5 nor be less than 0.67.

Panels within a special segment shall either be all Vierendeel panels or all X-braced panels; neither a combination thereof nor the use of other truss diagonal configurations is permitted. Where diagonal members are used in the special segment, they shall be arranged in an X-pattern separated by vertical members. Diagonal members within the special segment shall be made of rolled flat bars of identical sections. Such diagonal members shall be interconnected at points where they cross. The interconnection shall have a required strength equal to 0.25 times the nominal tensile strength of the diagonal member. Bolted connections shall not be used for diagonal members within the special segment.

Splicing of chord members is not permitted within the special segment nor within one-half the panel length from the ends of the special segment.

The required axial strength of the diagonal web members in the special segment due to dead and live loads within the special segment shall not exceed $0.03 F_{y} A_{g} / \alpha_{s}$,

where

$A_{g}=$ gross area of the diagonal web members, in. ${ }^{2}\left(\mathrm{~mm}^{2}\right)$

E4.4b Stability Bracing of Trusses

Each flange of the chord members shall be laterally braced at the ends of the special segment. The required strength of the lateral brace shall be determined as follows:

(E4-1)

where $\begin{aligned} A_{f} & =\text { gross area of the flange of the special segment chord member, } \mathrm{in} .^{2}\left(\mathrm{~mm}^{2}\right) \\ F_{y} & =\text { specified minimum yield stress of special segment chord member, } \mathrm{in} .^{2}\left(\mathrm{~mm}^{2}\right) \\ R_{y} & =\text { ratio of expected yield stress to the specified minimum yield stress of special } \\ & \text { segment chord member, } F_{y}\end{aligned}$

E4.4c Stability Bracing of Truss-to-Column Connections

The columns shall be laterally braced at the levels of top and bottom chords of the trusses connected to the columns. The required strength of the lateral braces shall be determined as follows:

(E4-2)

where

• $\begin{aligned} P_{n c} & =\text { nominal axial compressive strength of the chord member at the ends, kips } \\ & \text { (N) }\end{aligned}$
• $R_{y}=$ ratio of expected yield stress to the specified minimum yield stress of chord member, $F_{y}$

E4.4d Stiffness of Stability Bracing

The required brace stiffness shall meet the provisions of Specification Appendix 6, Section 6.2, where

(E4-3)

E4.5a Basic Requirements

Columns shall satisfy the requirements of Section D1.1 for highly ductile members.

E4.5b Special Segment Members

The available shear strength of the special segment shall be calculated as the sum of the available shear strength of the chord members through flexure and of the shear strength corresponding to the available tensile strength and 0.3 times the available compressive strength of the diagonal members, when they are used. The top and bottom chord members in the special segment shall be made of identical sections and shall provide at least 25% of the required vertical shear strength.

The available strength of chord members, $\phi_{t} P_{n}$ (LRFD) and $P_{n} / \Omega_{t}$ (ASD), determined in accordance with the limit state of tensile yielding, shall be equal to or greater than 2.2 times the required strength, where

$\phi_{t}=0.90$ (LRFD) $\quad \Omega_{t}=1.67$ (ASD)

(E4-4)

where

$A_{g}=$ gross area of chord member, in. ${ }^{2}\left(\mathrm{~mm}^{2}\right)$

$F_{y}=$ specified minimum yield stress of chord member, ksi (MPa)

E4.5c Expected Vertical Shear Strength of Special Segment

The expected vertical shear strength of the special segment, $V_{e}$, at mid-length, shall be determined as follows:

(E4-5)

where

• $I \quad=$ moment of inertia of a chord member of the special segment, in. ${ }^{4}\left(\mathrm{~mm}^{4}\right)$
• $L \quad$ = span length of the truss, in. (mm)
• $L_{s} \quad$ = length of the special segment, in. (mm)
• $\begin{aligned} M_{n c} & =\text { nominal flexural strength of a chord member of the special segment, kip-in. } \\ & \text { (N-mm) }\end{aligned}$
• $P_{n c}=$ nominal axial compressive strength of a diagonal member of the special segment, kips (N)
• $P_{n t}=$ nominal axial tensile strength of a diagonal member of the special segment, kips (N)
• $R_{y}=$ ratio of expected yield stress to the specified minimum yield stress of the chord member or diagonal member, $F_{y}$, as applicable
• $\alpha \quad$ = angle of diagonal members with the horizontal, degrees

E4.5d Width-to-Thickness Limitations

Chord members and diagonal web members within the special segment shall satisfy the requirements of Section D1.1b for highly ductile members. The width-to-thickness ratio of flat bar diagonal members shall not exceed 2.5.

E4.5e Built-Up Chord Members

Spacing of stitching for built-up chord members in the special segment shall not exceed 0.04 $E r_{y} / F_{y}$, where $r_{y}$ is the radius of gyration of individual components about their minor axis.

E4.5f Protected Zones

The region at each end of a chord member within the special segment shall be designated as a protected zone meeting the requirements of Section D1.3. The protected zone shall extend over a length equal to 2 times the depth of the chord member from the connection with the web members. Vertical and diagonal web members from end-to-end of the special segments shall be protected zones.

E4.6a Demand Critical Welds

The following welds are demand critical welds and shall satisfy the requirements of Sections A3.4b and I2.3:

• (a) Groove welds at column splices
• (b) Welds at column-to-base plate connections

Exception: Welds need not be considered demand critical when both of the fol- lowing conditions are satisfied.

• (1) Column hinging at, or near, the base plate is precluded by conditions of restraint.
• (2) There is no net tension under load combinations including the overstrength seismic load.

E4.6b Connections of Diagonal Web Members in the Special Segment

The end connection of diagonal web members in the special segment shall have a required strength that is at least equal to the expected yield strength of the web member, determined as $R_{y} F_{y} A_{g} / \alpha_{s}$,

where

$A_{g}=\text { gross area, in. }^{2}\left(\mathrm{~mm}^{2}\right)$

$F_{y}=$ specified minimum yield stress, ksi (MPa)

$R_{y}=$ ratio of expected yield stress to the specified minimum yield stress, $F_{y}$

E4.6c Column Splices

Column splices shall comply with the requirements of Section E3.6g.