What are Wood-Armer moments? How can we view those?
Wood Armer method allows moment triads from plates (Mx, My, Mxy) to be transformed into simple bending moments in two directions (Wood Armer moments) for reinforcement design of plate elements. This is important because the twisting moment Mxy can be significant.
At any point in the slab, the moment normal to a direction, resulting due to design moments Mx, My, and Mxy, must not exceed the ultimate normal resisting moment in that direction. The ultimate normal resisting moment is typically provided by ultimate resisting moments Mux and Muα related to the reinforcement in the x- and α- directions. Mx, My and Mxy are bending and twisting moments, usually obtained from a finite element analysis program. α is angle of transverse steel, measured clockwise, from the Mx axis.
Now, Wood Armer moments are available for plate elements only and for these too, the reinforcement direction needs to be defined. The plate elements could either be created using create element option or by using the Mesh option as shown in the image above. After that, click on “Define Domain” as indicated in image above and this will invoke the define domain dialogue box as shown on the right. By defining the domain, plates which form the slab or raft can be assigned. To assign the plates to a particular domain, follow the steps as shown here:
1. Enter the domain name
2. Select the element type
3. Select the required plates in the model or directly enter the element numbers as shown
4. Click on Add
Now, to add sub domain, one could either click on the Sub-Domain button as shown in the “Define Domain” dialogue box or click on define sub domain as shown in the ribbon menu image at the top right. The sub-domain is where the specifics for the group of plates are entered. This includes details like the angle of reinforcement to the global axis and angle between the reinforcements spanning in either direction as well as type of member. The basic reinforcement which is to be considered in each direction at top and bottom can also be specified in the sub domain. If for different parts of the same domain have different details, then these could be provided using the subdomain.
On applying the necessary loads and boundary conditions, the Wood Armer moments could be checked graphically as well as in form of table. For an irregular hypothetical slab as shown below, the Wood Armer moment result in the direction of reinforcement (i.e., the design moment) will be more than those in local X and Y directions of the plate.
Along the red line shown in the image above, the Mxx value obtained for its dead load was 62kNm, while the Wood Armer moment, considering the Mxx, Myy and Mxy, which would all be acting along the direction of reinforcement, i.e., at 20o angle to the global X direction is 74.2kNm. This is the moment for which the slab needs to be designed.
The Wood Armer moments could be obtained at top and bottom of plates along both the reinforcement directions. This option could be accessed by clicking on the Wood Armer Moment button as shown in the image to the left.
The results could be viewed in tabular format by browsing to Results> Results Tables> Plate> Force (Unit Length)… Once the table interface opens, click on Plate Force (UL:W-A Moment as in image above and select the required load cases as well as output positions.