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001775

Cisca Tjoa, PE

2023-12-05

Cálculo de un pórtico arriostrado según AISC 341 en RFEM 6

El cálculo de un pórtico ordinario arriostrado concéntricamente (OCBF) y un pórtico especial arriostrado concéntricamente (SCBF) se puede llevar a cabo en el complemento Cálculo de acero de RFEM 6. El resultado del cálculo sísmico según AISC 341-16 y 341-22 se clasifica en dos secciones: Requisitos de barras y requisitos de conexiones.

En el artículo se tratan más detalles en profundidad sobre la entrada de configuración sísmica Kb | Cálculo sísmico AISC 341 en RFEM 6 .

Member Requirements

The following design checks for members that are part of the seismic force-resisting system (SFRS) are available in RFEM. The sections listed refer to Seismic Provisions AISC 341-16/22 [1].

Width-to-Thickness Limitations [Section D1.1 and F1.5a]
SCBF Stability Bracing of Beams – Required Strength & Stiffness [Sections F2.4b and D1.2a.1(b)]
SCBF Stability Bracing of Beams – Maximum Spacing [Sections F2.4b and D1.2a.1(c)]
Column Required Strength [Section D1.4a]
Brace Slenderness Ratio [Section F1.5b for OCBF and F2.5b(a) for SCBF]

Width-to-Thickness Limitations for Ductility Requirements

The braces in OCBF are designated as moderately ductile members according to Section F1.5a. All members (braces, beams, columns) in SCBF are designated as highly ductile members according to Section F2.5a.

The braces in OCBF must satisfy the requirements of AISC Seismic Provisions Section D1.1 for moderately ductile members. As an exception according to Section F1.5a, braces in tension-only frames with L_c/r greater than 200 do not need to satisfy the ductility requirement. This design check is shown as EQ 1300 in RFEM (Image 1).

KB 001775 | Cálculo de un pórtico arriostrado según AISC 341-16 en RFEM 6

Comprobación de ductilidad

Nota: Tension-only frames are not permitted in SCBF as per Section F2.4d.

Stability Bracing of Beams in SCBF

The requirement for stability bracing is only applicable for beams in V- and inverted V-braced frames as per Section F2.4b [1]. The required strength and stiffness of the stability bracings are listed in the Stability Bracing by Member tab under “Seismic Requirements” (Image 2). These values can be compared to the calculated available strength and stiffness when designing the bracing members that frame into the beam. There are no design check details available (only references).

Arriostramiento de estabilidad de vigas

The required strength, Pbr, is defined in Equation A-6-7 of Appendix 6 of AISC 360-16/22 [3]]:

$P_{br} = 0.02 \cdot (\frac{M_{r} \cdot C_{d}}{h_{o}})$

P_br	Resistencia necesaria del arriostramiento de la viga de estabilidad
M_r	Resistencia necesaria a flexión de la viga. M_r = R_y F_y Z/ α_s [AISC 341 Ecuación D1-1]
C_d	Factor de doble curvatura = 1,0 [AISC 341, sección D1.2a(b)]
h_o	Distancia entre el centro de gravedad del ala h_o = d - t_f

Arriostramiento de estabilidad de vigas (resistencia necesaria)

Nota: Pr is not applicable to braced frames.

The required stiffness, βbr, is defined in Equation A-6-8 of Appendix 6 of AISC 360-16/22 [3]:

$β_{br} = \frac{1}{Φ} \cdot (\frac{10 \cdot M_{r} \cdot C_{d}}{L_{br} \cdot h_{o}}) (LRFD)$

$β_{br} = Ω \cdot (\frac{10 \cdot M_{r} \cdot C_{d}}{L_{br} \cdot h_{o}}) (ASD)$

d	Canto útil
d	Profundidad estática eficaz
d	Profundidad estática eficaz
f_y	Límite elástico del acero de refuerzo
d	Profundidad estática eficaz

Arriostramiento de estabilidad de las vigas (rigidez necesaria)

The maximum spacing of the stability bracing must satisfy the requirements of AISC 341-16/22 Section F2.4b, which refers to Section D1.2a.1(c):

$L_{br} = \frac{0.19 \cdot r_{y} \cdot E}{R_{y} \cdot F_{y}} (AISC 341 - 16)$

$L_{br} = \frac{0.17 \cdot r_{y} \cdot E}{R_{y} \cdot F_{y}} (AISC 341 - 22)$

L_br	Separación máxima del arriostramiento de la viga de estabilidad
r_y	Radio de giro respecto al eje débil
E	Módulo de elasticidad
R_y	Relación entre el límite elástico esperado y el límite elástico mínimo especificado
F_y	Mínima tensión de fluencia especificada

Arriostramiento de estabilidad de vigas para SCBF (separación máxima)

The design check for the maximum spacing is presented together with the other member requirements in the Design Ratios on Members. The braced length, Lb, is the specified effective length for lateral-torsional buckling (LTB). The design check detail is shown in EQ 2100 (Image 3).

Detalles de verificación

Column Required Strength

All columns that are part of the seismic force-resisting system (SFRS) are required to be designed with overstrength loads. In many cases, the amplified axial force does not need to be combined with the concurrent bending moments. The option to neglect all bending moments, shear, and torsion in columns for overstrength limit state is activated by default. This option can be deactivated in the Seismic Configuration.

For standard load combinations without overstrength from seismic load effect, the combined loading is checked according to AISC 360-16/22 Chapter H.

For load combinations with overstrength seismic load, Chapter H is not applied when the option to neglect all bending moments, shear, and torsion in columns for overstrength limit state is activated. Example 4.3.2 of the Seismic Design Manual [2] demonstrates the design using the controlling case from both load combinations, standard and overstrength.

Bending moments resulting from a load applied between points of lateral support can contribute to column buckling. Therefore, they are required to be considered concurrently with the axial loads by deactivating the option to neglect the moments (Image 4).

Resistencia necesaria del pilar

Brace Slenderness Ratio

For braces in V or Inverted V Frames in OCBF, the slenderness ratio L_c/r must be less than or equal to 4*√(E/F_y) according to Section F1.5b [1]. The intent is to limit the unbalanced forces that develop in framing members after brace buckling. This design check is shown as EQ 3300 in RFEM (Image 5).

For braces in X-configuration, the option to meet this requirement can be deactivated in the Seismic Configuration.

OCBF Comprobación de esbeltez de arriostramiento

For braces in SCBF, the slenderness ratio L_c/r must be less than or equal to 200, according to Section F2.5b(a) [1]. This design check is shown as EQ 3310 in RFEM (Image 6).

SCBF Comprobación de esbeltez de la riostra

Connection Requirements

The Seismic Requirements include the Required Connection Tensile Strength and the Required Connection Compressive Strength of the brace. They are listed in the Brace Connection by Member tab (Image 7). The design check details are not available for the connection strengths. However, the equations and standard references are listed in the table.

Conexión de riostras por barra

The symbols and definitions are shown in the following formulas:

$R_{y} \cdot F_{y} \cdot A_{g} / α_{s}$

R_y	Relación entre el límite elástico esperado y el límite elástico mínimo especificado
F_y	Mínima tensión de fluencia especificada
A_g	Área bruta de la riostra
α_s	Factor de ajuste del nivel de fuerza LRFD-ASD = 1,0 para LRFD y 1,5 para ASD

Resistencia a tracción necesaria de la conexión de riostras (OCBF y SCBF)

$1.1 \cdot F_{cre} \cdot A_{g} / α_{s} for OCBF (341 - 16)$

$1.14 \cdot F_{cre} \cdot A_{g} / α_{s} for SCBF (341 - 16 & 341 - 22)$

$F_{ne} \cdot A_{g} / α_{s} for OCBF (341 - 22)$

F_cre	Tensión crítica de pandeo de la sección E de AISC 360-16 utilizando el límite elástico esperado, R_y F_y
A_g	Área bruta de la riostra
α_s	Factor de ajuste del nivel de fuerza LRFD-ASD = 1,0 para LRFD y 1,5 para ASD
F_ne	Tensión nominal de la sección E de AISC 360-22 utilizando el límite elástico esperado, R_y F_y

Resistencia a compresión necesaria de la conexión de riostras

Nota: The capacity-limited design based on the expected braced strengths will be available in the future release.

Base de datos de conocimientos

KB 001775 | Cálculo de un pórtico arriostrado según AISC 341 en RFEM 6

Autor

Cisca es responsable del soporte técnico al cliente y el desarrollo continuo de los programas para el mercado norteamericano.

Enlaces

Kb | Cálculo sísmico AISC 341 en RFEM 6

Referencias

Instituto Americano de Construcción de Acero. (2016). AISC 341-16, Seismic Provisions for Structural Steel Buildings. Chicago: AISC.
AISC. (2018). Manual de diseño sísmico , (3ª ed.). Instituto Americano de Construcción de Acero, Chicago.
Instituto Americano de Construcción de Acero. (2016) Especificación para edificios de acero estructural , ANSI/AISC 360-16. Chicago: AISC.
Instituto Americano de Construcción de Acero (2022). Seismic Provisions for Structural Steel Buildings, AISC 341-22. Chicago: AISC.
Instituto Americano de Construcción de Acero (2022). Specification for Structural Steel Buildings, ANSI/AISC 360-22. Chicago: AISC.

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