【问题描述】
新版SFA中如何考虑覆土荷载和水荷载对基础的不利情况？
【答案详解】
程序允许用户提供土层面标高、水位标高以及基础底面标高。基于这些输入信息，参照基础底面标高来确定土层面标高和水位标高，以便计算基础所受荷载。
程序提供了以下选项，用于考虑土和水荷载的最不利影响情况，以供基础设计时考虑：
软件将根据覆土荷载和水荷载在程序内部生成的：
a. 水压力
b. 水压力（浮力）
c. 干土
d. 饱和土
e. 饱和土 + 干土
f. 淹没土 + 干土
g. 淹没土 + 饱和土
根据对这些荷载的选择情况，程序会利用这些荷载创建新的荷载组合。
示例：
- LC 01 : 1.2 DL + 1.4 LL + 1.4 Eq-X
如果在基础设计中用户选择了上述所有土和水荷载，那么覆土荷载和水荷载将添加到恒荷载项中，且程序将会创建以下荷载组合：
- LC 01 : 1.2 DL + 1.4 LL + 1.4 Eq-X + 1.2 Soil (Water Pressure)
- LC 01 : 1.2 DL + 1.4 LL + 1.4 Eq-X + 1.2 Soil (Water Pressure (Buoyancy))
- LC 01 : 1.2 DL + 1.4 LL + 1.4 Eq-X + 1.2 Soil (Dry)
- LC 01 : 1.2 DL + 1.4 LL + 1.4 Eq-X + 1.2 Soil (Saturated)
- LC 01 : 1.2 DL + 1.4 LL + 1.4 Eq-X + 1.2 Soil (Saturated + Dry)
- LC 01 : 1.2 DL + 1.4 LL + 1.4 Eq-X + 1.2 Soil (Submerged + Dry)
- LC 01 : 1.2 DL + 1.4 LL + 1.4 Eq-X + 1.2 Soil (Submerged + Saturated)
软件根据用户所选的情况，可以分成考虑/不考虑浮力作用的土荷载 ，用户可以根据下述各种情形来计算基础上方的土重：
其中：
P = 基础顶部/底部来自柱/墩台的轴向荷载（包括柱/墩台的自重）
D = 基础埋深
Dw = 地下水位深度
Ds = 土层深度
Fa = 基础面积
Cs = 柱/墩台横截面面积
- Case 1
地下水位高于基础埋深，但低于土层顶面。水位以上的土体是干燥的。不考虑浮力的影响。 由于未考虑毛细作用，水位以上的土体被视作干燥状态。
针对此情况需选择的选项: Saturated + Dry.
- Soil Pressure calculation:
Soil Pressure = (Dw – D) x γsat + (Ds-Dw) x γdry - Soil load calculation:
Soil load = (Dw – D) x γsat x (Fa – Cs) + (Ds-Dw) x γdry x (Fa – Cs) - Foundation self-weight pressure calculation:
foundation S/W pressure= D x γconc - Foundation S/W calculation:
Concrete load= D x Fa x γconc
Total Load on foundation = P + Soil load + Concrete load
- Case 2
地下水位高于基础埋深，但低于土层顶面。水位以上的土体是干燥的。考虑浮力的影响。
水位以上的土体被视作干燥状态，是因为未考虑毛细作用。
针对此情况需选择的选项: Submerged + Dry
- Soil Pressure calculation:
Soil Pressure = (Dw – D) x γsub + (Ds-Dw) x γdry - Soil Weight calculation:
Soil load= (Dw – D) x （γsat-γw） x (Fa – Cs) + (Ds-Dw) x γdry x (Fa – Cs)=（Dw-D）xγwx（Fa-Cs） - Foundation self-weight pressure calculation:
foundation S/W pressure= D x γconc - Foundation S/W calculation:
Concrete load= D x Fa x γconc - Water Upward force calculation:
Water weight= γw x (Fa x D) + γw x Cs x (Dw - D)
Total Load on foundation = P + Soil load + Concrete load - Water weight
- Case 3
地下水位高于基础埋深，但低于土层顶面。水位以上的土体是饱和的。不考虑浮力的影响。
由于毛细作用，水位以上的土体被视作饱和状态。
针对此情况需选择的选项: Fully Saturated
- Soil Pressure calculation:
Soil Pressure = (Ds – D) x γsat - Soil load calculation:
Soil load = (Ds – D) x γsat x (Fa – Cs) - Foundation self-weight pressure calculation:
foundation S/W pressure= D x γconc - Foundation S/W calculation:
Concrete load= D x Fa x γconc
Total Load on foundation = P + Soil load + Concrete load
- Case 4.
地下水位高于基础埋深，但低于土层顶面。水位以上的土体是饱和的。考虑浮力的影响。
由于毛细作用，水位以上的土体被视作饱和状态。
针对此情况需选择的选项:Submerged + Saturated.
- Soil Pressure calculation:
Soil Pressure = (Dw – D) x γsub + (Ds-Dw) x γsat - Soil Weight calculation:
Soil load= (Dw – D) x γsub x (Fa – Cs) + (Ds-Dw) x γsat x (Fa – Cs) - Foundation self-weight pressure calculation:
foundation S/W pressure= D x γconc - Foundation S/W calculation:
Concrete load= D x Fa x γconc - Water Upward force calculation:
Water weight= γw x (Fa x D) + γw x Cs x (Dw - D)
Total Load on foundation = P + Soil load + Concrete load - Water weight
- Case 5
地下水位低于基础底面标高。
针对此情况需选择的选项: Fully Dry
- Soil Pressure calculation:
Soil Pressure = (Ds – D) x γdry - Soil Weight calculation:
Soil load= (Ds – D) x γdry x (Fa – Cs) - Foundation self-weight pressure calculation:
foundation S/W pressure= D x γconc - Foundation S/W calculation:
Concrete load= D x Fa x γconc - Water Upward force calculation:
Water weight= NA
Total Load on foundation = P + Soil load + Concrete load
注意：即便水位高于基础顶面，如果你选择计算干土重量（完全干燥状态），程序会假定地下水位低于基础底面标高来计算土的重量。
- Case 6
地下水位高于基础埋深，但基础上方无土层。
针对此情况需选择的选项: Submerged + Dry or Submerged + Saturated.
- Soil Pressure calculation:
Soil Pressure = NA - Soil Weight calculation:
Soil load= NA - Foundation self-weight pressure calculation:
foundation S/W pressure= D x γconc - Foundation S/W calculation:
Concrete load= D x Fa x γconc - Water Upward force calculation:
Water weight= γw x (Fa x D) + γw x Cs x (Dw - D)
Total Load on foundation = P + Concrete load - Water weight
- Case 7
土层标高低于基础底面标高，而水位高于基础底面标高。不考虑浮力。
针对此情况需选择的选项:Saturated + Dry or Fully Saturated.
- Soil Pressure calculation:
Soil Pressure = NA - Soil Weight calculation:
Soil load= NA - Foundation self-weight pressure calculation:
foundation S/W pressure= D x γconc - Foundation S/W calculation:
Concrete load= D x Fa x γconc - Water Downward force calculation:
Water weight= (Dw -D) x γw x (Fa – Cs)
Total Load on foundation = P + Concrete load + Water weight (acting downward向下作用)
- Case 8
地下水位高于土层标高（水淹情况）。
针对此情况需选择的选项:(Submerged + dry) +/- Buoyancy.
- Soil Pressure calculation:
Soil Pressure = (Ds – D) x γsat - Soil Weight calculation:
Soil load= (Ds – D) x γsat x (Fa – Cs) - Foundation self-weight pressure calculation:
foundation S/W pressure= D x γconc - Foundation S/W calculation:
Concrete load= D x Fa x γconc - Water Upward force calculation:
Water weight= γw x (Fa x D) + γw x Cs x (Dw - D)
Total Load on foundation = P + Soil load + Concrete load - Water weight
注意：
如果要将土层标高之上的水重视作作用在基础上的向下荷载，该程序允许创建一个单独的荷载工况来施加这类荷载。