Dimensionamento de um pórtico contraventado segundo a AISC 341 no RFEM 6

Para mais detalhes sobre a entrada de configuração sísmica, consulte o artigo KB | Dimensionamento sísmico AISC 341 no RFEM 6 .

Requisitos da barra

As seguintes verificações de dimensionamento para barras que fazem parte do sistema resistente a forças sísmicas (SFRS) estão disponíveis no 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)]
• Resistência necessária do pilar [secção D1.4a]
• Brace Slenderness Ratio [Section F1.5b for OCBF and F2.5b(a) for SCBF]

Limites de largura-espessura para os requisitos de ductilidade

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).

Verificação de ductilidade

Atenção: 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). Estes valores podem ser comparados com a resistência e rigidez disponíveis calculadas quando dimensionam as barras de contraventamento que formam um pórtico na viga. Não estão disponíveis detalhes da verificação de dimensionamento (apenas referências).

Contraventamento de estabilidade de vigas

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

Atenção: 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]:

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):

The design check for the maximum spacing is presented together with the other member requirements in the Design Ratios on Members. O comprimento contraventado Lb, é o comprimento efetivo especificado para encurvadura por flexão-torção (LTB). The design check detail is shown in EQ 2100 (Image 3).

Detalhes de verificação

Resistência do pilar necessária

All columns that are part of the seismic force-resisting system (SFRS) are required to be designed with overstrength loads. Em muitos casos, a força axial aumentada não necessita de ser combinada com os momentos de flexão actuais. A opção para negligenciar todos os momentos fletores, cortes e torções nos pilares para o estado limite da sobrerresistência está ativada por defeito. Esta opção pode ser desativada na configuração para sismos.

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.

Os momentos fletores resultantes de uma carga aplicada entre os pontos de apoio laterais podem contribuir para a encurvadura do pilar. Therefore, they are required to be considered concurrently with the axial loads by deactivating the option to neglect the moments (Image 4).

Resistência necessária do 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.

Verificação de esbelteza do contraventamento OCBF

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).

Verificação da esbelteza do contraventamento SCBF

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.

Ligação de contraventamento por barra

The symbols and definitions are shown in the following formulas:

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

Base de dados de conhecimento

KB 001775 | Dimensionamento de um pórtico contraventado segundo a AISC 341 no RFEM 6