## Question

**I want to perform a detailed analysis of a slab to check the unit length shear force Mxx value. In Plate Force Options, when should Element and Avg. Nodal be used?**

## Answer

**The Element option is used in areas where the flow of force is interrupted, while Avg. Nodal is used in areas where the flow of force is not interrupted.**

To explain with specific examples, let's consider a slab subjected to gravity loads and another subjected to lateral loads. For slabs subjected to gravity loads, it's generally advisable to use the Avg. Nodal option to determine member forces. However, if the slab is resisting lateral forces, then the Element option should be used to evaluate member forces.

For example, let's compare the member forces for a slab subjected to gravity loads and another subjected to lateral loads, as shown in image1 and image 2.

1. When checking Mxx for the slab subjected to gravity loads, there's not much difference between the values obtained using the Element and Avg.Nodal options.

2. When checking Mxx for the slab subjected to lateral loads, a significant difference is observed between the values obtained using the Element and Avg.Nodal options.

The reason for this difference is that, in Figure 4(a), comparing the Mxx on the left and right sides of the slab around the column's axis reveals that they are almost equal in magnitude and opposite in sign. Since Avg. Nodal calculates the average unit length shear moment of elements connected to column intersections, it results in distorted values and is thus not suitable.

[Image] Slab subjected to gravity loads [Image] Slab subjected to lateral loads

(a) Using the Element option (b) Using the Avg. Nodal option

[Image] Unit length shear force Mxx for the slab subjected to gravity loads

(a) Using the Element option (b) Using the Avg. Nodal option

[Image] Unit length shear force Mxx for the slab subjected to lateral loads

(a) Under gravity loads (b) Under lateral loads

[Image] Distribution of unit length shear force Mxx for the slab