Расчёт деревянных балок-колонн по NDS 2018 с помощью дополнительного модуля RF-/TIMBER AWC

Рассматриваемый стержень изготовлен из южной сосны № 2 номинальным размером 2х4, длиной 3 фута и используется в качестве элемента фермы. Lateral support is provided only at the member ends, and they are considered pinned. The dead (DL), snow (SL), and wind (WL) loads are applied at the top and midpoint of the beam-column, as shown below.

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Деревянная колонна

Характеристики стержня отображаются при выборе соответствующего сечения и материала в программе.

Сечения пиломатериалов 2х4

Свойства материала южной сосны

Поправочные коэффициенты для расчета допускаемых напряжений (ASD), указанные в таблице 4.3.1 NDS 2018

Исходные расчетные значения (F_b, F_c и E_min) умножаются на соответствующие поправочные коэффициенты для нахождения скорректированных расчетных величин. For sawn lumber, these factors are found in Table 4.3.1 [1]. Для расчета методом ASD существует одиннадцать различных поправочных коэффициентов. Many of these factors are equal to 1.0 in the NDS example [2]. Не смотря на это, мы кратко поясним далее способ учета отдельных коэффициентов в модуле RF-/TIMBER AWC.

Factors Calculated by Program

C_L – Beam Stability Factor

It depends on the geometry and lateral support of the member as described in Section 3.3.3 [1]. Модуль RF-/TIMBER рассчитывает данный коэффициент автоматически.

Note: the effective length, le, used to calculate C_L is defined by the user in the "Effective Length" section of RF-/TIMBER AWC. The "Acc. to Table 3.3.3" option with the appropriate loading case must be selected.

Загружение, примененное в данном примере, показано на рисунке ниже.

Полезная длина для коэффициентаC L

C_F – Size Factor

It depends on the depth and thickness of the member as specified in Section 4.3.6 [1]. Модуль RF-/TIMBER рассчитывает данный коэффициент автоматически.

C_fu – Flat Use Factor

It accounts for weak axis bending of the member as specified in Section 4.3.7 [1]. Коэффициент рассчитывается в модуле RF-/TIMBER AWC автоматически.

C_P – Column Stability Factor

It depends on the geometry, end fixity conditions, and lateral support of the member as described in Section 3.7.1 [1]. When a compression member is fully supported throughout its length, C_P = 1.0. This factor is automatically calculated in RF-/TIMBER AWC for both strong and weak axis directions.

Factors Defined by User Input

C_D – Load Duration Factor

It accounts for various loading periods based on the load case, such as dead, snow, and wind, based on Section 4.3.2 [1]. Selecting "ASCE 7-16 NDS (Wood)" as the standard in RFEM activates the load duration option in the Load Cases dialog box. The load duration class (Permanent, Ten Years, and so on) default setting is based on the "Action Category" of the load case. Данный параметр может быть изменен пользователем в программе RFEM или RF-/TIMBER AWC. The value selected by the program is based on Table 2.3.2 [1].

C_M – Wet Service Factor

It accounts for the moisture service conditions of the member as specified in Section 4.1.4 [1]. В модуле RF-/TIMBER AWC пользователь может выбрать «влажные» или «сухие» условия в столбце «Условия эксплуатации».

C_t – Temperature Factor

It accounts for exposure to elevated temperatures of up to 100 degrees F, 100 to 125, and 125 to 150 as described in Section 2.3.3 [1]. The user can select between the three temperature ranges in the "In-Service Conditions" section of RF-/TIMBER AWC. The value selected by the program is based on Table 2.3.3 of [1].

C_i – Incising Factor

It accounts for the loss of the area from the small incisions made in the member to receive preservative treatment for decay prevention as described in Section 4.3.8 [1]. The user can select "Not Incised" or "Incised" in the "Additional Design Parameters" section of RF-/TIMBER AWC.

C_r – Repetitive Member Factor

It is used when multiple members act compositely to properly distribute a load amongst themselves as described in Section 4.3.9 [1]. C_r = 1.15 for members that meet the criteria of being closely spaced and connected by a sheathing or equivalent. The user can select "Not Repetitive" or "Repetitive" in the "Additional Design Parameters" section of RF-/TIMBER AWC.

Внимание! If necessary, code-based values of the user input adjustment factors can be changed in the "Standard" option.

Возможность поправочных коэффициентов

Factors Excluded in Program

C_T – Buckling Stiffness Factor

It accounts for the contribution of plywood sheathing to the buckling resistance of compression truss chords as specified in Section 4.4.2 [1]. Этот коэффициент используется для увеличения значения E_min стержня. C_T can be manually calculated as per <nobr>Equation 4.4-1 [1]</nobr> or conservatively taken as 1.0.

C_b – Bearing Area Factor

It is used to increase the compression design values (F_cp) for concentrated loads applied perpendicular to the grain as specified in Section 3.10.4 [1]. C_b can be manually calculated per equation 3.10-2 [1] or conservatively taken as 1.0.

Actual Stress in Beam-Column

В этом примере сочетание нагрузок упрощено до CН1: DL + SL + WL.

• Compression stress from dead and snow loads, f_c = 171 psi
• Изгибающее напряжение вокруг оси максимальных моментов от ветровой нагрузки f_bx = f_b1 = 353 фунт/кв.дюйм
• Weak-axis bending stress from dead and snow loads, f_by = f_b2 = 1,029 psi

Determination of Adjusted Design Values as per NDS 2018 Table 4.3.1 ASD Method

• Critical Buckling Design Value for Compression Member in Strong Axis, F_cEx:
  $${\mathrm{F}}_{\mathrm{cEx}} = {\mathrm{F}}_{\mathrm{cE}1} = \frac{0.822 · {\mathrm{E}}_{\min }\text{'}}{{\left[{\displaystyle \frac{{\mathrm{l}}_{\mathrm{e}1}}{{\mathrm{d}}_1}}\right]}^2}$$

  $${\mathrm{F}}_{\mathrm{cEx}} = {\mathrm{F}}_{\mathrm{cE}1} = \frac{0.822 · 510,000 \mathrm{psi}}{{\left[{\displaystyle \frac{36.0 \mathrm{in}}{3.5 \mathrm{in}}}\right]}^2}$$

  $${\mathrm{F}}_{\mathrm{cEx}} = {\mathrm{F}}_{\mathrm{cE}1} = 3,963 \mathrm{psi}$$

  FcEx	Critical buckling design value for the compression member in the major axis, psi
  Emin'	= Emin ⋅ CM ⋅ CT ⋅ Ci = 510,000 psi
  le1	Effective length = 36.0 in
  d1	Member depth = 3.5 in

  Расчетное значение критической потери устойчивости при изгибе сжатого стержня вдоль главной оси, FcEx.
• Critical Buckling Design Value for Compression Member in Weak Axis, F_cEy:
  $${\mathrm{F}}_{\mathrm{cEy}} = {\mathrm{F}}_{\mathrm{cE}2} = \frac{0.822 · {\mathrm{E}}_{\min }\text{'}}{{\left[{\displaystyle \frac{{\mathrm{l}}_{\mathrm{e}2}}{{\mathrm{d}}_2}}\right]}^2}$$

  $${\mathrm{F}}_{\mathrm{cEy}} = {\mathrm{F}}_{\mathrm{cE}2} = \frac{0.822 · 510,000 \mathrm{psi}}{{\left[{\displaystyle \frac{36.0 \mathrm{in}}{1.5 \mathrm{in}}}\right]}^2}$$

  $${\mathrm{F}}_{\mathrm{cEy}} = {\mathrm{F}}_{\mathrm{cE}2} = 728 \mathrm{psi}$$

  FcEy	Critical buckling design value for the compression member in the minor axis, psi
  Emin'	= Emin ⋅ CM ⋅ CT ⋅ Ci = 510,000 psi
  le2	Effective length = 36.0 in
  d2	Member thickness = 1.5 in

  Расчетное значение критической потери устойчивости при изгибе сжатого стержня вдоль второстепенной оси, FcEy.
• Adjusted Compressive Design Value Parallel to Grain, F_c':
  $${\mathrm{F}}_{\mathrm{c}}\text{'} = {\mathrm{F}}_{\mathrm{cy}}\text{'} = {\mathrm{F}}_{\mathrm{c}} · {\mathrm{C}}_{\mathrm{D}} · {\mathrm{C}}_{\mathrm{M}} · {\mathrm{C}}_{\mathrm{t}} · {\mathrm{C}}_{\mathrm{F}} · {\mathrm{C}}_{\mathrm{i}} · {\mathrm{C}}_{\mathrm{P}}$$

  $${\mathrm{F}}_{\mathrm{c}}\text{'} = {\mathrm{F}}_{\mathrm{cy}}\text{'} = 1,450 \mathrm{psi} · 1.6 · 1.0 · 1.0 · 1.0 · 1.0 · 0.29$$

  $${\mathrm{F}}_{\mathrm{c}}\text{'} = {\mathrm{F}}_{\mathrm{cy}}\text{'} = 673 \mathrm{psi}$$

  Fc'	Adjusted compressive design value parallel to the grain, psi
  Fc	Reference compressive design values parallel to the grain, psi
  CD	Load duration factor
  CM	Wet service factor
  Ct	Temperature factor
  CF	Size factor
  Ci	Incising factor
  CP	Column stability factor

  Скорректированное расчетное значение сжатия в направлении волокон, Fc'.
• Critical Buckling Design Value for Bending Member, F_bE:
  $${\mathrm{F}}_{\mathrm{bE}} = \frac{1.20 · {\mathrm{E}}_{\min }\text{'}}{{{\mathrm{R}}_{\mathrm{B}}}^2}$$

  $${\mathrm{F}}_{\mathrm{bE}} = \frac{1.20 · 510,000 \mathrm{psi}}{{9.65}^2}$$

  $${\mathrm{F}}_{\mathrm{bE}} = 6,577 \mathrm{psi}$$

  FbE	Critical buckling design value for the bending member, psi
  Emin'	= Emin ⋅ CM ⋅ CT ⋅ Ci = 510,000 psi
  RB	Slenderness ratio = 9.65 < 50 (NDS Equation 3.3-5)

  Расчетное значение критической потери устойчивости изгибаемого стержня, FbE.
• Adjusted Strong Axis Bending Design Value, F_bx':
  $${\mathrm{F}}_{\mathrm{bx}}\text{'}= {\mathrm{F}}_{\mathrm{b}1} = {\mathrm{F}}_{\mathrm{b}} · {\mathrm{C}}_{\mathrm{D}} · {\mathrm{C}}_{\mathrm{M}} · {\mathrm{C}}_{\mathrm{L}} · {\mathrm{C}}_{\mathrm{t}} · {\mathrm{C}}_{\mathrm{F}} · {\mathrm{C}}_{\mathrm{i}} · {\mathrm{C}}_{\mathrm{r}}$$

  $${\mathrm{F}}_{\mathrm{bx}}\text{'}= {\mathrm{F}}_{\mathrm{b}1} = 1,100 \mathrm{psi} · 1.6 · 1.0 · 0.982 · 1.0 · 1.0 · 1.0 · 1.0$$

  $${\mathrm{F}}_{\mathrm{bx}}\text{'}= {\mathrm{F}}_{\mathrm{b}1} = 1,729 \mathrm{psi}$$

  Fbx'	Adjusted major axis bending design value, psi
  Fb	Reference bending design value, psi
  CD	Load duration factor
  CM	Wet service factor
  CL	Beam stability factor
  Ct	Temperature factor
  CF	Size factor
  Ci	Incising factor
  Cr	Repetitive member factor

  Скорректированное расчетное значение изгиба по главной оси, Fbx'.
• Adjusted Weak Axis Bending Design Value, F_by':
  $${\mathrm{F}}_{\mathrm{by}}\text{'} = {\mathrm{F}}_{\mathrm{b}2} = {\mathrm{F}}_{\mathrm{b}} · {\mathrm{C}}_{\mathrm{D}} · {\mathrm{C}}_{\mathrm{M}} · {\mathrm{C}}_{\mathrm{L}} · {\mathrm{C}}_{\mathrm{t}} · {\mathrm{C}}_{\mathrm{fu}} · {\mathrm{C}}_{\mathrm{F}} · {\mathrm{C}}_{\mathrm{i}} · {\mathrm{C}}_{\mathrm{r}}$$

  $${\mathrm{F}}_{\mathrm{by}}\text{'} = {\mathrm{F}}_{\mathrm{b}2} = 1,100 \mathrm{psi} · 1.6 · 1.0 · 1.0 · 1.0 · 1.1 · 1.0 · 1.0 · 1.0$$

  $${\mathrm{F}}_{\mathrm{by}}\text{'} = {\mathrm{F}}_{\mathrm{b}2} = 1,936 \mathrm{psi}$$

  Fby'	Adjusted minor axis bending design value, psi
  Fb	Reference bending design value, psi
  CD	Load duration factor
  CM	Wet service factor
  CL	Beam stability factor
  Ct	Temperature factor
  Cfu	Flat use factor
  CF	Size factor
  Ci	Incising factor
  Cr	Repetitive member factor

  Скорректированное расчетное значение изгиба вокруг второстепенной оси, Fby'.
• Combined Biaxial Bending and Axial Compression Design Ratio

Inserting the actual stresses and limiting design values presented above into NDS equation 3.9-3 [1], the final design ratio is shown below.

And NDS equation 3.9-4 [1],

Результат в RF-/TIMBER AWC

The user can compare each adjustment factor and adjusted design value from the analytical hand calculation method to the result summary in RF-/TIMBER AWC. Как показано, результаты идентичны. The controlling final design ratio = 0.98 is based on the geometrically linear analysis (1^st degree) calculation method. Keep in mind that the default setting in RFEM for the load combination is set to the second-order analysis. This will result in a slightly larger design ratio = 1.03. The user has the option to choose which method listed in the "Calculation Parameters" is best for the structure.

Расчетное соотношение

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Колонна из деревянных балок

KB 001714 | Расчет деревянных колонн по норме NDS 2018 в модуле RF-/TIMBER AWC