Clean up sections

docs/Example/Structural/Statics/rc-section-analysis.md

@@ -14,8 +14,9 @@ The section configuration is shown as follows.
 
 ## Model Development
 
-The `SingleSection2D` element is **NOT** a connector element. Only one node is required to define the element. First we
-define an arbitrary node.
+The [`SingleSection2D`](../../../Library/Element/Special/SingleSection.md) element is **NOT** a connector element.
+Only one node is required to define the element.
+First we define an arbitrary node.
 
 ```
 node 1 0 0
@@ -101,4 +102,21 @@ is about $$350~\text{kNm}$$.
 
 ![rotation versus moment](rc-section-analysis.svg)
 
-Users with relative background may help to justify the result.
+Users with a relative background may help to justify the result.
+
+### Asymmetric Layout
+
+If the layout is asymmetric, say, for example, the rebars at $$y=-220~\text{mm}$$ are removed.
+
+```text
+section Rectangle2D 2 400. 500. 1 9
+section Bar2D 3 900. 2 220.
+# section Bar2D 4 900. 2 -220.
+section Bar2D 5 600. 2 0.
+```
+
+In this case, a positive moment makes the unreinforced region in tension.
+This decreases the moment capacity.
+However, a negative moment does not change the moment capacity significantly.
+
+Interested readers can try to apply both positive and negative rotation and verify the results.

docs/Library/Element/Beam/Orientation.md

@@ -7,8 +7,10 @@ Similar configurations are also required in other FEM software, see,
 for example, Section 26.3.4 `Beam element cross-section orientation` of the Abaqus Analysis User's Manual.
 
 The local $$x$$-axis is along the cord of the beam element.
-The local $$z$$-axis is often the strong axis of the section if there is one.
+The local $$z$$-axis is temporarily taken as the vector assigned by user.
 The local $$y$$-axis is defined to be the cross product of the local $$z$$-axis and the local $$x$$-axis.
+After obtaining the local $$y$$-axis, the local $$z$$-axis is redefined to be the cross product of the local $$x$$-axis
+and the local $$y$$-axis.
 
 Depending on the different literature and textbooks, different conventions may be used.
 Thus, the term local $$z$$-axis may not be the same axis you may expect.

docs/Library/Section/Section2D/Bar2D.md

@@ -2,12 +2,14 @@
 
 2D Reinforcing Bar Section
 
+![arrangement](PIC/Cell.svg)
+
 ## Syntax
 
 ```
 section Bar2D (1) (2) (3) [4]
 # (1) int, unique section tag
 # (2) double, area
 # (3) int, material tag
-# [4] double, eccentricity, default: 0.0
+# [4] double, eccentricity/location, default: 0.0
 ```

docs/Library/Section/Section2D/Box2D.md

@@ -2,6 +2,8 @@
 
 2D Box Section
 
+![arrangement](PIC/Box.svg)
+
 ## Syntax
 
 ```
@@ -12,11 +14,14 @@ section Box2D (1) (2) (3) (4) (5) [6] [7]
 # (4) double, thickness
 # (5) int, material tag
 # [6] int, number of integration points, default: 6
-# [7] double, eccentricity, default: 0.0
+# [7] double, eccentricity/location, default: 0.0
 ```
 
 ## Remarks
 
 The eccentricity is measured from the centre of the section.
 
 The width and height are measured from midline to midline.
+
+Under the hood, the section is computed as an I-section.
+Integration scheme applies.

docs/Library/Section/Section2D/Cell2D.md

@@ -0,0 +1,15 @@
+# Cell2D
+
+2D Cell Section
+
+![arrangement](PIC/Cell.svg)
+
+## Syntax
+
+```
+section Cell2D (1) (2) (3) [4]
+# (1) int, unique section tag
+# (2) double, area
+# (3) int, material tag
+# [4] double, eccentricity/location, default: 0.0
+```

docs/Library/Section/Section2D/Circle2D.md

@@ -2,6 +2,8 @@
 
 2D Circle Section
 
+![arrangement](PIC/Circle.svg)
+
 ## Syntax
 
 ```
@@ -10,7 +12,7 @@ section Circle2D (1) (2) (3) [4] [5]
 # (2) double, radius
 # (3) int, material tag
 # [4] int, number of integration points, default: 6
-# [5] double, eccentricity, default: 0.0
+# [5] double, eccentricity/location, default: 0.0
 ```
 
 ## Remarks

docs/Library/Section/Section2D/CircularHollow2D.md

@@ -2,6 +2,8 @@
 
 2D Hollow Circular Section
 
+![arrangement](PIC/CircularHollow.svg)
+
 ## Syntax
 
 ```
@@ -10,8 +12,8 @@ section CircularHollow2D (1) (2) (3) (4) [5] [6]
 # (2) double, radius
 # (3) double, thickness
 # (4) int, material tag
-# [5] int, number of integration points, default: 10
-# [6] double, eccentricity, default: 0.0
+# [5] int, number of integration points for half circle, default: 6
+# [6] double, eccentricity/location, default: 0.0
 ```
 
 ## Remarks

docs/Library/Section/Section2D/HSection2D.md

@@ -2,15 +2,17 @@
 
 2D H-Section
 
+![arrangement](PIC/HSection.svg)
+
 ## Syntax
 
 ```
 section HSection2D (1) (2...7) (8) [9] [10]
 # (1) int, unique tag
 # (2...7) double, section dimensions
 # (8) int, material tag
-# [9] int, number of integration points, default: 6
-# [10] double, eccentricity measured from the centre, default: 0.0
+# [9] int, number of integration points for flanges, default: 6
+# [10] double, eccentricity/location, default: 0.0
 ```
 
 ## Remarks
@@ -23,7 +25,10 @@ section HSection2D (1) (2...7) (8) [9] [10]
    * `(6)` web width
    * `(7)` web thickness
 
-The eccentricity is measured from the centre of the web.
+Two integration points are used for web.
+The user assigned number of integration points is used for flanges.
+
+The eccentricity is measured from the centre of the web/flanges.
 If the left and right flanges have different sizes, the default trivial eccentricity will lead to extra bending moments
 when the section is subjected to axial force.
 Eccentricity can be manually adjusted.

docs/Library/Section/Section2D/ISection2D.md

@@ -2,27 +2,32 @@
 
 2D I-Section
 
+![arrangement](PIC/ISection.svg)
+
 ## Syntax
 
 ```
 section ISection2D (1) (2...7) (8) [9] [10]
 # (1) int, unique tag
 # (2...7) double, section dimensions
 # (8) int, material tag
-# [9] int, number of integration points, default: 6
-# [10] double, eccentricity measured from the centre, default: 0.0
+# [9] int, number of integration points for web, default: 6
+# [10] double, eccentricity/location, default: 0.0
 ```
 
 ## Remarks
 
 1. Section dimensions are:
-   * `(2)` top flange height
+   * `(2)` top flange width
    * `(3)` top flange thickness
-   * `(4)` bottom flange height
+   * `(4)` bottom flange width
    * `(5)` bottom flange thickness
-   * `(6)` web width
+   * `(6)` web height
    * `(7)` web thickness
 
+Two integration points are used for both flanges.
+The user assigned number of integration points is used for web.
+
 The eccentricity is measured from the centre of the web.
 If the top and bottom flanges have different sizes, the default trivial eccentricity will lead to extra bending moments
 when the section is subjected to axial force.