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# Why does a 'Singular Error' occur while modeling beam piles or beam columns on a plate foundation?

## Question

Why does a 'Singular Error' occur while modeling beam piles or beam columns on a plate foundation?

Since beam and plate elements have different degrees of freedom(d.o.f), there can be a 'Singular Error' when you connect those elements directly. Then the model should be modified using the Rigid Link function.

When structural modeling is performed using different elements, it is necessary to avoid direct connections between elements having different d.o.f.

For example, while a beam element has 6 degrees of freedom (Dx, Dy, Dz, Rx, Ry and Rz), the plate element has 5 degrees of freedom except for the rotational d.o.f (Rx) which is torsion. Therefore, as shown in the figure below, if a torsional load is applied to the beam element, the red node where the beam and plate elements meet cannot resist the torsional load. It shall rotate, causing a Singular Error at the adjacent node to the connection node during analysis.

As shown above, when a Singular Error occurs at the connection node of different elements, the model should be modified using the Rigid Link function.

Rigid Link is a function that makes the Slave Node dependent on the d.o.f of the Master Node. Therefore, if the connecting node is set as a master node and the adjacent node as a slave node, then the slave node is dependent on the master node's d.o.f and has the Rx degree of freedom. As a result, the singular error does not occur during analysis.

It is recommended to select all the noes of plate elements that correspond to the cross-sectional area of the beam elements as the slave nodes when modeling the rigid link as shown in the figure above.

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