1846x

001775

Cisca Tjoa, PE

2023-12-05

Progettazione di strutture controventate secondo AISC 341 in RFEM 6

La verifica di un telaio ordinario controventato concentricamente (OCBF) e di un telaio speciale concentricamente controventato (SCBF) può essere eseguita nell'add-on Verifica acciaio di RFEM 6. Il risultato della verifica sismica secondo AISC 341-16 e 341-22 è classificato in due sezioni: Requisiti delle aste e requisiti di collegamento.

I dettagli più approfonditi sull'input della configurazione sismica sono trattati nell'articolo Kb | Verifica sismica AISC 341 in 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 | Progettazione di strutture controventate secondo AISC 341-16 in RFEM 6

Verifica di duttilità

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).

Controvento di stabilità delle travi

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	Resistenza richiesta del controvento della trave di stabilità
M_r	Resistenza a flessione richiesta della trave. M_r = R_y F_y Z/ α_s [AISC 341 Equazione D1-1]
C_d	Coefficiente di doppia curvatura = 1.0 [AISC 341 sezione D1.2a(b)]
h_o	Distanza tra il baricentro dell'ala h_o = d - t_f

Controvento di stabilità delle travi (resistenza richiesta)

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	Altezza efficace
[CRASHREASON.DESCRIPTION]	Altezza efficace
[CRASHREASON.DESCRIPTION]	Altezza efficace
f_y	Tensione di snervamento dell'acciaio di armatura
d	Altezza efficace

Controvento di stabilità delle travi (rigidità richiesta)

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	Spaziatura massima del controvento della trave di stabilità
r_y	Raggio di inerzia intorno all'asse debole
E	Modulo di elasticità
R_y	Rapporto tra la tensione di snervamento prevista e la tensione di snervamento minima specificata
F_y	Tensione di snervamento minima specificata

Controvento di stabilità delle travi per SCBF (spaziatura massima)

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).

Dettagli della verifica

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).

Resistenza richiesta della colonna

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.

Controllo della snellezza del controvento 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).

Controllo della snellezza del controvento 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.

Collegamento del controvento per asta

The symbols and definitions are shown in the following formulas:

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

R_y	Rapporto tra la tensione di snervamento prevista e la tensione di snervamento minima specificata
F_y	Tensione di snervamento minima specificata
A_g	Area lorda del controvento
α_s	Coefficiente di regolazione del livello di forza LRFD-ASD = 1.0 per LRFD e 1.5 per ASD

Resistenza a trazione del collegamento del controvento richiesta (OCBF e 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	Tensione di instabilità critica dalla sezione E di AISC 360-16 utilizzando la tensione di snervamento prevista, R_y F_y
A_g	Area lorda del controvento
α_s	Coefficiente di regolazione del livello di forza LRFD-ASD = 1.0 per LRFD e 1.5 per ASD
F_ne	Tensione nominale dalla sezione E di AISC 360-22 utilizzando la tensione di snervamento prevista, R_y F_y

Resistenza a compressione del collegamento del controvento richiesto

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

Knowledge Base

KB 001775 | Progettazione di strutture controventate secondo AISC 341 in RFEM 6

Autore

Cisca è responsabile dell'assistenza tecnica ai clienti e dello sviluppo continuo del programma per il mercato nordamericano.

Link

Kb | Verifica sismica AISC 341 in RFEM 6

Bibliografia

Istituto americano per le costruzioni in acciaio. (2016). AISC 341-16, Seismic Provisions for Structural Steel Buildings. Chicago: AISC.
AISC. (2018). Manuale di progettazione sismica , (3a ed.). American Institute of Steel Construction, Chicago.
Istituto americano per le costruzioni in acciaio. (2016) Specifiche per edifici in acciaio strutturale , ANSI/AISC 360-16. Chicago: AISC.
American Institute of Steel Construction (2022). Seismic Provisions for Structural Steel Buildings, AISC 341-22. Chicago: AISC.
American Institute of Steel Construction (2022). Specification for Structural Steel Buildings, ANSI/AISC 360-22. Chicago: AISC.

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