3 Westgate Street, Southery.

PE38 0PA. United Kingdom.

studio@anthony-johnson-engineering.co.uk:

Telephone

01945 660504

An Engineering Practice:

Based in the Heart of the Fens, North of Cambridge, Anthony-Johnson offers Structural, Geotechnical, Civil and Environmental Engineering services to Homeowners, Architects, Builders and Businesses in Wisbech, Kings Lynn, Downham Market, Thetford and surrounding areas

Tag Archive

Traditional Construction and longspan floorjoist design

The practice was tasked with the structural design of new floor joists for an existing Georgian building. The aim of the design was to span floor joists from solid internal masonry partition to exernal masonry wall, thereby allowing for the removal of an existing steel downstand beam that cut right through the large room, reducing the ceiling height and encroaching on the existing headroom.

Interestingly, in designing the new floor, it soon became apparent that a span of 5 meters would result in joists 75mm wide x 225mm deep, just to remove the excessive deflection and resultant vibration in the first floor. An existing roll topped bath, set freestanding to the first floor bathroom would have otherwise caused a “camber” in the floor as it was filled and emptied.

The attic space incorporated a series of large water tanks, supported directly off an internal partition, running from first floor ceiling, down to ground floor ceiling and existing joists, spanning onto the steel beam, carried all this load. A key issue was therefore the mass of the masonry on the joists and the loads applied by the water tanks. Re-location of the water tanks was a viable option and the masonry partition, discerned as being free-standing would require demolition, with timber stud-work a suitable replacement.

Timber cut roof to stately home

Design of joists to the location of the roll top bath was limited to “doubled up” sections, picking up the extra loads imposed by the cast iron tub and water it contained.

CAD drawing of new floor joist layout

Finally, there was the matter of joist hangers, as the client wanted to “retrofit” the joists from the ground floor, limiting the damage that would otherwise occur in the existing first floor bathroom. Opting for Simpson Masonry joist hangers was clearly the most suitable solution and they were designed to fit into slots, cut into the masonry, so that new joists could be sited in between existing ones.

Simpson Masonry joist hangers

Grade II* listed Georgian Property in a Conservation Area

The North Brink in Wisbech is synonymous with Georgian Architecture, much of which was designed and commissioned by the Peckover Family. I was contracted by the owners of North Brink Medical Practice as Structural Engineer to a project that would see the internal re-modeling of the third floor to this historic structure. The work involved was considerable and involved:

  • Removal of the existing Plaster and Lathe Ceiling
  • Installation of a new Ceiling at a height that allowed full use of the new space.
  • Removal of an internal single skin brick partition

An initial site visit allowed me to carry out an inspection of the existing ceiling timbers and notably, two of these formed ties to the wall plate, thereby preventing the spread of the walls due to roof loads. The ties also acted as main structural supports, carrying the ceiling loads themselves.

My key concern when carrying out the designs for this project was to retain the existing historic features of the structure as far as possible. This meant limiting structural alterations so that the original fabric of the building remained intact.


The construction techniques used in a building of this age clearly dictate it’s longevity. Timbers were included in the construction of it’s external walls, providing some “elasticity” that would otherwise not be present in a building with walls formed from “bricks and mortar”. The existing masonry still retains it’s original lime mortar with pozzolan clearly defined against the white of the lime on freshly “opened up” faces. Timbers were ungraded at the time of construction but appear denser than those used in modern construction today. Lead flashing is thicker than the gauges employed in roofing today. Movement and true levels in the building itself,  whilst clear against straight edges, is lost through the use of skilled construction techniques and use of traditional building materials.

In designing the new ceiling, it was important to check that the existing timber rafters could accommodate the new loads imposed by new joists that would be attached to them. Vertical loading at the connections could quite feasibly cause undue sagging of the roof. Purlins spanning perpendicular to the rafters could be subjected to greater stresses in bending and these needed to be checked. Loads on these were determined using the “Stiffness Matrix” method, whereby the rafters were analyzed for spanning over four supports.

Interestingly, the new joists were slender about their minor axis, so I introduced Noggins between them to prevent buckling. This had the added benefit of making the new connections between the joists and rafters act in shear, taking out the risk of the timbers failing at the new connections. With stress in the timbers being within tolerable limits, a final design was drawn up that would inadvertently strengthen the existing structure for years to come!

However, the removal of the internal wall, which clearly was not carry any vertical load proved an eye opener to all those involved in the project. In carrying out wind load calculations, it soon became apparent that the wall did indeed carry loads- horizontally! In effect, the central wall, formed from a single skin of brickwork, acted as a support to the main external wall when buffeted by wind from the South. It prevented the wall from bowing out as the wind tried to “suck” at it and took load when the wind tried to “push” on it. My solution to this was to design a single column, fixed to the spine wall of the building below and rafters above. Formed from a Structural Hollow section (SHS), the column was fixed back to the wall at every fourth brick course and acts as a mid-span support to the wall itself.

Structural Design:

A client was in the process of extending his property with a bespoke Brick and timber single storey extension. Having produced preliminary drawings and specifications and obtained both Planning and Building regulations approval, he felt that a Vaulted Ceiling would be more in keeping with the property than a trussed roof. I was engaged to design a steel beam over a series of bi-fold doors, allowing for a ridge beam to run the full length of the extension, thereby supporting rafters and a series of velux windows set into the roof.

In designing this new roof, I had to consider the entire structure, including rafters, ridge beam, steel post to the ridge beam and steel beam over the bi-fold doors. I first analyzed loading on the roof to Eurocodes, taking into account snow loading and drifted snow loading, wind on the dominant face of the building and Dead (permanent) loads from self weight. The roof, even with the load from concrete tiles would be subject to Uplift, so wall ties running the length of the wall were necessary.

Designing the timber members to the roof meant considering deflection in the members first- the finishes were brittle and included glass!. They were then checked to Eurocode 5 to make sure they could take the design load.

The steelwork beam and post was again designed for deflection and checked for the Ultimate Limit State. This time, I wanted to make sure the bi-fold doors continued to work even when the weather had turned poor during the winter!

Although not a complicated design, it’s important to note the need to consider deflection of structural members as the critical design factor in this project. I could have quite easily used smaller structural members but deflection in excess of span/360 would have meant redecorating within 5 years and doors and velux roof lights that were forever sticking!