3 Westgate Street, Southery.

PE38 0PA. United Kingdom.



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Richard Anthony Johnson


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Historic Conservation & Structural Engineering

Structural design for Historic Buildings can be a tricky beast. Traditional building materials introduce additional factors that need to be taken into account during the design process. Consequently, structural calculations become a “secondary” checking process to other concerns, bourne out of a need to consider the fabric of the building and the intrinsic issues bourne out of how the building interacts with the environment and the long term stabilization of building materials.

Sag of the roof, pushing out on the top of the wall, a tie rod and ceiling binder pulling in the opposite direction and the wall splits about the weakest point- right next to a window opening.

A Large Barn conversion project, currently being undertaken in conjunction with the practice in Suffolk clearly reflects these issues. The barn, formed of timber framing, solid masonry dwarf walls and flint infill panels lends itself to a structure that was not built with the modern Building Regulations in mind. A lack of foundations, replicated through rubble fill and rammed earth, coupled with local drainage ditches has resulted in the rotation of dwarf masonry wall panels, with external flint facing panels introducing weak points through a lack of cross-bonding between adjacent bricks. Shrinkage of the underlying CLAY bearing strata has introduced movement which has been exacerbated by out-of-plane axial loads applied by the timber framing set to sole plates to the head of the dwarf walls. Thrust to the head of full height masonry wall panels by the cut timber roof has lead to parting of masonry panels, with rotation about their base.

Outward rotation of wall panel due to shrinkage of clay ground and out-of plane loads from Supported timber structure.

This in of itself would ordinarily result in a re-build. However, graded structures impart a limitation on the type of works that can be carried out. Direct replacement, like for like is not possible, unless formerly agreed with the Conservation officer and as such, an alternative has to be sought. In the case of the damaged wall panels, underpinning was considered a viable solution, with strips foundations designed, which would be formed from mass filled concrete, poured to excavations formed under the existing walls. Constructed in sections, reinforcement would be added to tie seach section together, ultimately forming a concrete strip under the whole wall panel. Building up from the strip, to the underside of the traditional wall would then be carried out with a Limecrete, thereby forming a barrier between the traditional structure and the modern forundation. It would also allow the foundation to be set at a depth that would permit the formation of a limecrete floor slab internally. Finally, Helifix reinforcing bars would be used to stitch the cracks in the traditional masonry, with lime mortar re-pointing reinstating the traditional finish.

Remedial Works to wall panel, with underpinning calculated with achieve suitable bearing pressure, at a depth where shrinkage of the clay is limited due to a lack of change in pore water pressures.

Environmental damage is significant, with timbers clearly having failed due to beetle infestation and moisture ingress over many years. Roof timbers have snapped in two, resulting in the complete collapse of many sections of the roof, allowing exposure of internal elements, leading to further decay and failure of structural elements.

Converting the structure for residential use will require significant re-building works. However, given that the building is grade II listed, its important to retain as much of the heritage as possible. As a result, design works need to be sympathetic, avoiding the direct replacement of structural elements, except where their function is paramount to the stability of the structure. For example, failed studs should be supplemented with additional supports, rather than directly replacing them, timbers should be spliced wherever necessary.

Failed rafters, rotten studs, wood worm all leading to a loss of structural strength and ultimately structural failure

However, there is the option to make good to repair works that have previously introduced defects into the structure. Prior re-building works with modern portland cement based mortars and modern brick sets have introduced hard surfaces and dis-continuoud joints within the masonry wall panels. Such areas can be broken out and re-built, keying in traditional brick sets with Lime Mortars, ensuring continuity to the wall panel, thus allowing loads to distribute evenly through the structure.

Modern Portland Cement based mortar and No Cross Bonding forming a weak plane in the masonry

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

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.

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.

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.

Buyers Survey

Last week, I was asked to carry out yet another Structural survey. Having completed the survey, I didn’t think it was worth repeating myself and putting anything up on the examples page. There were a few cracks to the property which certainly weren’t of concern.

One of the cracks, top the underside of a window followed a plane of weakness in the brickwork, where builders had inadvertently lined up bed joints vertically. The same crack appeared around brickwork to the top of the window.

However (and this is a big however), whilst visiting an event in Whittlesea, I happened upon a long freestanding wall (about 30 meters long) which had a vertical crack that had propagated the entire height of the wall.

That got me thinking- Had the addition of a new UPVC window on a South Westerly facing wall inadvertently introduced a plane of weakness in the wall itself, allowing a restraint crack to develop? This form of cracking is typical in a wall 30 meters in length but not very common in a detached bungalow.

It’s the reason why Building regulations specify the need for vertical movement joints in long walls- to allow for expansion and contraction due to changes in temperature!. However, the size of the opening for the window and the changing of the window frame to UPVC as opposed to timber could have caused restraint in the wall so that as it expanded and contracted, the weakest point in the wall failed. The issue with that hypothesis was that the crack was nearly 800mm from the edge of the window frame. However, it did happen to coincide with an internal partition that was in turn restrained by the chimney breast and a line of weakness in the brickwork itself.

Either way, the vertical alignment of mortar joints (see image right) clearly points to a vertical plane of weakness in the masonry. As can be seen in the image, a bed joint, one course down from the window cill is poorly pointed and this line of mortar has cracked and spalled. The crack runs horizontally right under the entire window and then down the wall. Thankfully in this instance, it’s clear from the brickwork below the Damp Proof Course (the bottom course is Engineering Bricks with DPC over) that there hasn’t been any movement in the foundations, meaning that this can be easily rectified with little extra cost.

Structural Survey & Renovation

I was recently engaged by a client to carry out the survey of a property for restoration. Purchased at auction, the property was in a significant state of decay, with previous building works partly completed. Sections of the property were at risk of collapse due to a lack of suitable temporary supports and other parts of the property’s interior had been constructed using inadequate structural beams for supports. A short schedule of works was drawn up as part of the survey and presented to the client, so that a builder could be contracted to rectify structural defects before the weather deteriorated. These works included the shoring up of an existing external wall, precariously supported on “cantilevered” Acro Props some 3 meters in length. The temporary supports, which were positioned incorrectly already showed signs of having moved under the weight of masonry they were supporting and a crack, 1 inch in width had opened up to the interior of the supported brickwork skin.

As a consequence of the survey, I advised the client to engage the services of an Architect, who would produce preliminary plans, against which I could produce Structural Calculations for internal steelwork beams, timber floor joists and masonry walls. Given that the roof to the property had been completely removed and new trusses fitted, tiles, battens and felt could not be laid until a suitable planning application was granted by Fenland District Council. This would cause significant delay to the commencement of any works on-site, leaving the existing building at risk from further decay, as it was not watertight. It was therefore imperative that a design, commensurate with the surrounding properties was produced, so as to facilitate a quick and painless planning approval, thereby allowing works on the property to commence in earnest.

Initial Layout/ Architectural Layout:

A client requested the initial design of a series of 3 dwelling units, designed to fall within the curtilage of an existing steel portal Framed Farm Building. Falling under the Guise of a Class Q Change of Use, a suitable design would be provided with “Deemed Consent”, automatically meeting the requirements of Planning.

The initial appraisal of the Portal Framed Barn took the form of a site visit and survey, so as to obtain existing section sizes of the steel members, the dimensions of the portal frame bays and dimensions of the Haunches at both Apex and Eaves.

An analysis of the structure to both BS5950 and Eurocode 3, revealed that the Portal frames could accommodate suitable loads from a cladding system that would provide adequate insulation. Sway was found to be L/200 and this dictated the initial layout of the retro-fitted interior.

Layout: The building’s overall residential area is constrained by Class Q to less than 450 square meters on plan.

Instep areas, employing a timber cladding system were devised, off-setting the external appearance of the Steel cladding. Large windows, allowing for extensive lighting were included, thereby allowing for the orientation of the building, which was found to be in a Northerly Direction.

Internal Masonry, comprising Block work was included to allow for disproportionate collapse of the building, thereby protecting occupants of adjacent dwelling units, should one of the units collapse due to fire or accidental damage.

The floor to the existing building was raised by 500mm so as to allow for the building being in a “Flood Zone 3”.

This property is located within the curtilage of a Growth Village, meaning that it stands a “better” chance of being granted “Change of Use” consent. The consent itself takes into account other factors other than external appearance.

This includes the vicinity to services including water, gas, sewage and electricity, the location or vicinity to “special” areas and whether the property or land has, within a given period, been used for Farming.

Property Survey:

A client requested my assistance with a Structural Engineers Survey of his Semi Detached Property. The property, which had been placed on the Market, had previously remained unsold, with a prospective buyer “pulling out” of the sale shortly after having received a Structural Engineers report on the property.

Prior knowledge of the report produced by a local Structural Engineer meant that I approached this survey with trepidation. Professional indemnity can inadvertently result in the Engineer being overly Cautious when carrying out a survey for a prospective buyer, thereby finding faults in the property when there are none to be found. In conducting this survey for the vendor, I surveyed the entire exterior of the property, finding little more than a covered Air vent to the underside of the floating timber floor to the front elevation of the property.

A Resin injected DPC to the rear was unfortunately installed to a solid brick wall and whilst correctly corresponding to an internal concrete ground bearing slab, would do little for a property of this age. However, further inspection revealed moss to the exterior face of the wall, extending up to the underside of the original DPC but no further, This indicated that masonry above the original DPC remained dry and free of internal moisture.

The interior of the property was similarly immaculate and the lack of cracks, damp, movement or dry rot within the property itself indicated that it was in sound condition.

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!