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The Design of Structural Steelwork for a House

Based North of Wisbech, just outside of Throckenholt, a client wanted some structural alterations to be designed for his property....

The Design of Structural Steelwork for a House

Based North of Wisbech, just outside of Throckenholt, a client wanted some structural alterations to be designed for his property. The design itself was extensive, requiring the removal of both internal and external load bearing walls, thereby opening up the property and introducing a new entrance route into the building itself.

Removal of the internal load bearing wall meant that a steel frame would have to be used in its place (image left). Simply introducing a steel beam would result in loads being transferred into outer walls and the foundations to the property were not considered adequate to accommodate the additional bearing pressures. Additionally, the existing wall that the steelwork would replace was there to support the outer wall as it resisted wind loading (you’ve probably seen sagging walls with steel plates in them- the sag it due to wind loading and the plate is there to tie it back to floor joists). Floor joists to the first floor spanned parallel to the outer wall in this location, so the horizontal span of the wall had to be kept low- hence the column provided some thing for it to “lean” against!

As a result, I had to design a “picture frame” which would act in the same way as the wall it was replacing. The top steel beam carried the loads from above, transferring them into columns and down into another beam, located on top of the existing footings, distributing vertical loads evenly over the length of the footings. Without the beam to the bottom, stresses would have been so high that by the time they reached the underside of the footings, they would have caused the ground to subside over time as it “consolidated” under the additional load.

A similar solution was designed for the first floor to the front of the property, allowing for complete removal of the first floor front wall. However, this particular steelwork was also designed to carry horizontal loads, imposed by wind loading to a new steel frame that would enclose the front of the house and be encased in a glazed finish. As a result, the “picture frame” steelwork replacing the old front wall had to be positioned so that the horizontal wind loads would be evenly distributed back into the roofs rafters and first floor timber floor joists.

Why didn’t I just design the glass front to be bolted onto the side elevations protruding out of the house? One of the biggest issues I had was that to the left of the house (above image), the extension was circa 100 years old and to the right circa 150 years old.

Construction techniques differed considerably during this era and both fronts could inadvertently move to different degrees when heated and cooled by sunlight. In effect, I was aware that there was a risk of “differential movement” and didn’t want the new steel glazed frame to suffer as a result.

Footings to the new steel frame had to be designed to avoid transferring loads into the existing corbel footings to the base of the existing house. In order to do this, I designed a reinforced concrete beam that would span over the top of the new footings, cantilevering out each end so that the steelwork could be positioned up against the existing house but all vertical loads would be carried by the new footings. A compressible membrane could be used in the void between new and existing, removing the risk of “foundation interaction”.

As can be seen in the image on the right, the foundations to the new steelwork appear to be deigned in two stages, with brickwork in between. This was intentional as it allows loads to be carried by the top reinforced concrete beam. These loads are then transferred through “bearing” into the brickwork, which in turn bears directly onto a 650mm wide strip footing founded 750 mm below ground level. The bearing pressure to the underside of this strip footing is less than 30kPa and at the depth shown will not suffer from any desiccation or swelling of the clay that it is founded on.