# 3.7.3.4 Shell Forces

With the **“Shell Forces”**-component one can retrieve principal or local shell cross section forces. Use the drop-down-list at the bottom of the component to switch between **“Principal”** and **“Local”**.

## **Principal Shell Forces**

Sometimes it is of interest to know the value of principal normal forces and moments on shells. In such cases the **“Shell Forces”**-component for principal forces comes in handy. The order of all result lists corresponds to the order of faces in the mesh used to generate the shell, which is the same order as the **“Principal Force Directions on Shells”**-component. Thus, the element centers returned there can be used in combination with the numeric results. The third branch index from the right is the shell element index.

<figure><img src="/files/ZNyHExJ0bzouzFNvBveZ" alt=""><figcaption><p>Fig. 3.7.3.4.1: Shell with one sub-element under three different load-cases - output of the envelope of the principal cross section forces.</p></figcaption></figure>

Distributed normal forces are negative in case of compression. Positive bending moments result in tension on the upper side of a shell. The upper side of a shell element is defined by a positive value of the local Z-axis. When in doubt about the orientation of your shell elements enable the preview of local element axes in the **“ModelView”**-component (see section [3.6.1](/3-in-depth-component-reference/3.6-results/3.7.1-general-results/3.6.1-modelview.md)). The output-plugs **“vx”** and **“vy”** return transverse shear forces along lines parallel to the shells local x- and y-axis.

Depending on the result selection one or more results are output. Fig. 3.7.3.4.1 shows the envelope of the principal cross section forces ('min' and 'max' are enabled). Since there is no inique result load-case for a face **"LCInd"** returns -1 to indicate that.

## **Local Shell Forces**

When setting the result type to **“Local”** the shell cross section forces in local coordinate directions will be output. See section [3.1.14](/3-in-depth-component-reference/3.1-model/3.1.14-orientate-element.md#orientate-shell) on how to change the local coordinate systems of shells.

<figure><img src="/files/018CmiS9jvvDbsXwbc5c" alt=""><figcaption><p>Fig. 3.7.3.4.2: Shell with one sub-element under three different load-cases - output of the three corresponding local cross section forces.</p></figcaption></figure>

Fig. 3.7.3.4.2 shows the retrieval of the local distributed cross section forces for all load-cases of the load-case combination "LCC" (neither 'min' nor 'max' are selected). **"LCInd"** returns now the index of the load-case within the load-case combination which corresponds to the cross-section forces.

{% file src="/files/VZE46ylAEzcHhgVjh4XA" %}


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