Portal Frames and Sway:

We have seen that the supports of a continuous or fixed beam may move vertically due to settlement of foundations, etc. As they are statically indeterminate structures these support movements introduce internal stress resultants in them. In portal frames, further to vertical support movements, another type of joint movement is possible which is in the horizontal direction and is known as sway. That sway movement is also capable of affecting the internal stress resultants in the portal members, and hence cannot be ignored. However, not all portals are subjected to sway. Look at Figure.

The portal ABCD is subjected to a horizontal load B, hence, the joint B and C having freedom to move towards the right may occupy the position, B′ and C′ as shown by dotted lines. The movements BB′ (which is same as CC′) will be called the amount of sway at this level BC. However, in Figure 17(b), the portal is held at joint C to the fixed support E by another member CE. This will not allow free sway of C (or B). Hence, this portal will be without any sway. In a multistoreyed portal, independent sways are possible at each 'floor' or 'storey' level. However, we are not discussing here multi-storeyed portals.