Более подробная информация о вводе сейсмической конфигурации описана в отдельной статье. КБ | Сейсмический расчёт по AISC 341 в программе RFEM 6 .
Требования к стержням
В программе RFEM имеются следующие виды расчетов для стержней, являющихся частью устойчивой к сейсмической нагрузке системы (SFRS). Перечисленные пункты относятся к нормам по сейсмостойкости AISC 341-16 [1].
- Ограничения ширины к толщине [п. D1.1]
- Связи устойчивости балок - требуемая прочность и жесткость [пп. D1.2a.1(b) для IMF и D1.2b для SMF]
- Связи устойчивости балок - максимальный шаг [пп. D1.2a.1(c) для IMF и D1.2b для SRF]
- Связи устойчивости балок в местах расположения шарниров - требуемая прочность [п. D1.2c.1(b)]
- Требуемая прочность колонны [п. D1.4a]
- Коэффициент гибкости колонны для незакрепленного соединения [Сечение E3.4c.2b]
Width-to-Thickness Limitations for Ductility Requirements
Members in IMF are designated as moderately ductile members according to Section E2.5a. Members in SMF are designated as highly ductile members according to Section E3.5a.
Полка колонны
The column flange of SMF must satisfy the requirements of AISC Seismic Provisions Section D1.1 [1] for highly ductile members. This design check is shown as EQ 1200 in RFEM (Image 1).
Стенка колонны
The limiting width-to-thickness ratio for webs of highly ductile members is determined using the governing load case for axial load, as stipulated in Section D1.4a [1]. The governing load case is based on all load combinations, including gravity only CO, CO with standard seismic load, and CO with overstrength seismic load. This check is shown in EQ 1100 in RFEM (Image 2).
Similar to the columns, the width-to-thickness checks are also done for the beams.
Придание жесткости для устойчивости балок
The required strength and stiffness of the stability bracings are listed in the Stability Bracing by Member tab under “Seismic Requirements” (Image 3). 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).
There are two different values listed for the required strengths. The first value, Pbr, is applicable for the stability bracings that are located outside the plastic hinge location. Pbr is defined in Equation A-6-7 of Appendix 6 of AISC 360-16 [3]:
| Pbr | Требуемая прочность связей устойчивой балки |
| Mr | Требуемая прочность балки на изгиб. Mr = Ry Fy Z/ αs [AISC 341 формула D1-1] |
| Cd | Коэффициент двойной кривизны = 1,0 [AISC 341, раздел D1.2a (b)] |
| ho | Расстояние между центром тяжести полки ho = d - tf |
The second, larger value, Pr, is specifically for the stability bracings at the plastic hinge location. It is defined in Equation D1-4 of AISC 341-16 [1]:
| Pr | Требуемая прочность связи устойчивой балки в месте расположения пластического шарнира |
| Ry | Отношение ожидаемого предела текучести к заданному минимальному пределу текучести |
| Fy | Заданный минимальный предел текучести |
| Z | Эффективный пластический момент сопротивления сечения (или соединения) в месте расположения пластического шарнира |
| αs | Коэффициент регулировки уровня силы LRFD-ASD = 1,0 для LRFD и 1,5 для ASD |
| ho | Расстояние между центром тяжести полки |
The required stiffness, βbr, is defined in Equation A-6-8 of Appendix 6:
| d | Расчетная высота |
| d | Расчетная высота |
| d | Расчетная высота |
| fy | Предел текучести арматурной стали |
| d | Расчетная высота |
The maximum spacing of the stability bracing must satisfy the requirements of AISC 341-16 Section D1.2a.1(c) for IMF and Section D1.2b for SMF.
| Lbr | Максимальный шаг связей устойчивой балки |
| ho | Расстояние между центрами тяжести полок |
| U | Удлинение |
| S (Ta ) | Расчетное спектральное ускорение для основного периода конструкции в рассматриваемом направлении |
| û | Перемещение узла |
The design check for the maximum spacing is presented together with the other member requirements under “Design Ratios of Members”. The design check detail is shown in EQ 2100 (Image 4). The braced length, Lb, is the specified effective length for lateral-torsional buckling (LTB).
Column Required Strength
All columns that are part of the seismic force-resisting system (SFRS) are required to be designed with the overstrength loads. Во многих случаях нет необходимости сочетать увеличенную нормальную силу с одновременными изгибающими моментами. 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, Chapter H.
For load combinations with overstrength seismic load, Chapters F and H are not checked when the option to neglect all bending moments, shear, and torsion in columns for overstrength limit state is activated.
In Example 4.3.2 of the seismic manual [2], the controlling case from both load combinations, standard and overstrength, needs to be considered.
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.
Column Slenderness Ratio for Unbraced Connection
For columns in SMF with no transverse member bracing at the connection, the potential for out-of-plane buckling at the connection shall be minimized by limiting the slenderness ratio L/r to be equal to or less than 60, according to Section E3.4c.2b [1]. Unbraced connections occur in special cases, such as in a two-story frame without an intermediate floor.
In all other cases, the option to meet this requirement can be deactivated in the Seismic Configuration.
The connection requirements are covered in the article КБ | Прочность соединения рам, устойчивых к моменту, по AISC 341-16 в RFEM 6 .
