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The Design of a Load Bearing Steel Beam

A client contracted the practice to design a steel beam, to act as a replacement of an existing load bearing wall...

The Design of a Load Bearing Steel Beam

A client contracted me to design a steel beam, to act as a replacement of an existing load bearing wall. In effect, a simple process of designing a beam and the pad-stones to either end, thereby providing support to the floor joists above in bending and transferring the same loads into the supporting walls through bearing. An initial design resulted in a 152 x 152 UC 23, spanning onto Engineering Bricks to either end, set in Cat II mortar. Design stressed in the existing Block work wall indicated that the pad-stone had to be 400mm wide in order to prevent crushing. 3 Courses of Engineering bricks would distribute loads from the underside of the beam so as to reduce the applied stresses sufficiently.    

However, having reviewed the existing structure, it soon became apparent that a window to the ground floor kitchen would inadvertently prevent the formation of a pad-stone to the cavity wall of the building. In effect, the concrete lintel over the window spanned at such a height and in such a location that it ended right in the middle of where I would locate the pad-stone. In addition, the lack of further partitions running perpendicular to the external wall was of concern. With the internal partition removed, the external wall would span some 7 meters, unrestrained and floor joists ran parallel to it. As a result, a pier would have to be formed from the internal block work wall, meaning that not all of it could be demolished. The second iteration of the design incorporated the pier, avoiding the need to form a pad-stone in the location of the Lintel but the stresses introduced in the pier by the steel beam would result in crushing of the concrete block work. At the same time, stresses to the underlying footings would not be evenly distributed and without sufficient information on allowable bearing capacities, the potential for failure in the footings had to be considered a possibility.

The resultant solution aimed to solve all these issues in as elegant way as possible. A steel hanger, formed from a 120 x 120 x 10 RSA was designed to sit within the internal leaf of the cavity wall. It would be inserted just above the line of the Lintel and an end plate would connect back to the steel beam. The RSA would have its downstand removed where it did not coincide with the beam, thereby allowing the wall to be re-rendered and plastered. Boxing in of the steel beam would also hide the remaining downstand and connection back to the beam, thereby leaving a clean finish once completed.

The Completed works: The left hand picture below shows a steel hanger protruding from the wall, carrying the steel beam. The right hand picture below shows the detail (above), with brick pier retained as a wind post. The steel beam is hung from the rolled steel angle (RSA) with a side plate welded onto it. M16 bolts fix the beam back to the side plates. Note that the wind post is formed from remains of the existing load bearing wall. This is important because the bricks need to be tied back (cross bonded) into the inner leaf of Gable end cavity wall. Without the cross bonding, the pier offers no resistance against wind loads.