Case Studies

STRUCTURAL STRENGTHENING & REPAIR WORKS AT NORTH FINCHLEY LIDO & LEISURE CENTRE

MODERN RESTORATION: DEVELOPING NEW METHODS WITH GLULAM BEAMS AND STRUCTURAL REPAIRS

Project role: Main Contractor

Project timeline: June 2019 – September 2020

June – September 2019: Pre-contract feasibility studies, structural reports, research & development of repair and restoration methodologies

October 2019 – September 2020: Phases I, II & III undertaken with continued R&D throughout the project

October 2020: Completion & Handover of project to the client, Barnet Council.

Project value: £1.2M

History of the site

The Finchley Open Air Pool was designed by P T Harrison and was officially opened on 26 March 1932, on 22 April the Duke of York (to become King George VI) unveiled a ceremonial wall tablet made of Staffordshire marble, which can be seen to this date on display behind the counter of Nando’s restaurant, which now occupies a plot built around the original site.

The whole site occupied 7 acres (28,000 m2), and there was a car park, with its own uniformed attendant (paid the equivalent of £3 per week in 1932). The car park was enlarged in 2004 when the original cinema on the north side of the site was demolished following large-scale flooding.

The main heated pool measured 165 ft (50 m) × 80 ft, depth ft 3 ins to 10 ft (3.0 m), with two fountains. This pool had a diving stage, containing a two-stage platform. Finchley Lido was the first swimming pool to introduce gay nudist days and events; this was a pilot scheme which later introduced trials in Manchester  and other boroughs in London.

On 11 July 1971, the Lido had 11,962 visitors, which remains the record highest attendance in one day for a UK lido

The original 1930s outdoor pool, was enclosed by an elegant colonnade of Roman Doric columns, with fountains to either side and was used to host the men’s preliminaries to the water polo competition for the 1948 Summer Olympics. The original site with its main pool and children’s pool was closed in 1992.

Pre-contract studies of the Modern site

North Finchley Lido & Leisure Centre as we know it today was constructed in 1996 and sits within the Great North Leisure Park built on the vast site of the former 1930’s built Finchley Lido. 

The modern lido and leisure centre was closed to the public in February 2019 after major structural defects were discovered during condition surveys. In a releventy short period of time since its construction the building’s supporting steel elements had suffered from extreme rust corrosion due to a combination of the harsh atmospheric conditions and inadequate air handling of the humid environment. 

The condition surveys prompted further structural reports commissioned by the client which provided enough detail to inform the client to make the decision to close the pools, although these initial reports contained enough basic information and were adequate to base our enabling works upon and eventually formed part of the tender documentation provided to us at phase I of the works, more research & development and extensive investigative works were required to provide granular information that would determine whether it would be cost effective to repair the building or demolish it and rebuild, needless to say our repair and restoration proposals and the associated cost forecasts were deemed financially viable to save the existing building. 

Phase I – Enabling works

Following the success of our bid to undertake Phase I of the project and prior to commencing with any remedial works, there was the initial phase of enabling works which involved the design of structural propping to the roof & glulam beams, full scaffolding access to allow  investigative works, further detailed surveys, feasibility studies, cost projections and reports, all of which had to be provided to the client and their professional team for approval before the contract for phase 2 remedial works could be awarded to Guild. 

Draining the pool:

To enable Phase I the pool had to be emptied, something in itself which is not without risk of causing damage. As the pressure of the water is released from the structure and it rapidly dries, usually for the very first time following its initial filling, therefore careful consideration has to be given to removing such volumes of weight and moisture from the structure in a short space of time, there are factors of contraction to consider. 

Being as risk averse as possible we explored the possibilities of the best and worst case scenarios, we anticipated that at best a number of tiles would debond from the walls, at worst;  the contraction of the pool structure would compromise the structural integrity of the walls and the walls of the pool would fail in multiple locations so the decision to drain the pool was not taken lightly,  however, in order to be able to install the required propping to take the roof loads of 15n  to ground we quickly persuaded the client to agree to draining the pool after exhausting all other options of transferring the roof loads elsewhere.

Once we had researched scenarios, all options, potential outcomes and had then developed a way forward, the contracts were drawn up and agreed upon we were then able to mobilise and take full possession of the site. 

It was subsequently discovered that the timber glulam beam roof and many of the supporting steel columns, steel box sections  which supported the 50M atrium and many of the connecting bolts had failed in their structural integrity due to the climatic and corrosive conditions of the swimming pool and were at high risk of imminent failure.

Structural propping

Following the structural engineer’s confirmation of the roof loads, Guild commissioned the design of the Mabey structural propping, it was required to independently support the roof loads as the apex connector plates needed to be completely removed in order to undertake the invasive install of the upgraded connections to the glulam beams, the propping design was integrated into and secured by the access scaffolding. 

Much consideration was given to the protection of the swimming pool floor so it was essential that was carefully planned, the conclusion reached was that the load bearing propping must be designed to be based on a series of specialist modular spreader plates and high strength flooring sheets to distribute the substantial roof loads and protect the fragile floor of the swimming pool. Upon completion of the strengthening works and removal of the propping and scaffolding this design proved to be a complete success with no damage whatsoever to the pool.

Phase II – Repairs to poolside

Strengthening works to Glulam beams

The glulam beams that form the roof by way of 2 adjoining beams each with a 25 meter span spread across 10 locations, were connected via an embedded inverted steel “T” and a circular 10mm steel plate with 6 No. M20 bolts at the apex and supported with steel columns at the eaves. All steel elements were suffering from progressive corrosion and required either complete replacement or where salvageable, abrasive blasting and recoating with marine grade paints was undertaken.

The existing connections; note the drop of the left beam and splits within the glulam beam

Glulam beams propped and ready for investigative works and surveys.

Plates and bolts removed showing the inverted “T” connector

The client’s project structural engineering team had designed a new apex connection for the 20 No. glulam beams which involved strengthening the 25 metre span of the beams by means of new stainless steel sleeves which were required to be pre cut off site but welded in situ. Guild commissioned and undertook a full and detailed condition survey of the glulam beams and based on that report and further liaison with the project structural engineers, Guild and their team of structural engineers and designers proposed a whole new repair design and methodology that involved the use of engineering resins and carbon fibre rods and flitch plates to repair the apex connections and resin injections to the numerous splits in the beams. 

With this proposal taken on board, Guild were then able to provide all of the structural calculations and performance data on the resins, adhesives and carbon fibre materials to the satisfaction of the client’s structural engineers’ approval, the client was happy to instruct us to proceed with the repairs and strengthening works based on our methodology.  In more practical terms regarding logistical issues and with regards to health & safety, we were able to mitigate the need for heavy lifting of stainless steel sleeves and hot works (welding) in the proximity of the glulam beams and roof soffit which in a building largely constructed of timber would have been a very high risk to manage. 

Design for the apex strengthening works showing embedded carbon fibre flitch plates, rods and location of structural resins and adhesives, new inverted T connectors were fabricated, the original steel steel plates were cleaned, recoated and reinstalled with new stainless steel bolts. 

Installing the strengthening carbon fibre rods at 45 degrees across the apex following the install of the embedded carbon fibre flitch plates.

Strengthening the structural failures with resin injections into the glulam beams.

Guild’s engineering team used specialist technical knowledge of structural timber repairs using specialist structural resins, adhesives and carbon fibre dowels and flitch plates,  our  invaluable input at the design stage of the project meant that the proposal was not only an improvement on the client’s initial design but it also offered a finish that is more aesthetically pleasing than the initially proposed large steel sleeves. 

Detail showing injection points to splits in the glulam beams

Refurbishment of the Atrium

The atrium had also become a source of water ingress into the building and ultimately another cause of rust as the gaskets and seals to the glass units had broken down and  failed so complete replacement of these was undertaken, a number of damaged glass units were also replaced.

During the replacement of the corroded horizontal steels which the atrium was set on, the substantial weight of the atrium frame and glazing required structural reinforcement so a load bearing scaffolding was designed and installed to transfer the loads from the roof and back to the Mabey propping.

Strengthening works to steel frame & support columns

corrosion to steel at high level

At roof level the horizontal steel box section beams supporting the 50 meter atrium were severely corroded at multiple points which seriously compromised their structural purpose as were the bolts securing them to the posts, which in turn was  transferring the load of the atrium onto the 25 meter span of glulam beams.

The Atrium box section steels and posts: Before & After replacement

Following the identification of the corroded structural elements, it was decided that where feasible, steel should be completely replaced, however where corrosion was serious but the integrity of the steels were satisfactory the existing coatings and rust were stripped and cleaned using an abrasive blasting system. Once cleaned to the specified SA 2.5 the in situ repair work could begin.

Before and after showing corrosion to the base of columns.

The surrounding concrete plinth was removed to expose the base plate for inspection, the plates were found to be in good condition and did not require replacement. Following abrasive cleaning to SA 2.5 new steel sleeves were installed over the existing to reinforce the columns, concrete plinths were reformed and finished with mosaic tiles as per the original.

Rust and coatings removed by means of abrasive blasting to SA 2.5 and finished in marine grade coatings suitable for the swimming pool environment.

Phase III – Repairs over the Gym & Fitness studios

The final phase of the project was particularly difficult to complete and presented a whole new set of problems to solve. Logistically this phase was the most difficult to undertake, due to the fact that the last sections of glulam beams were over the gym and fitness studios, which are situated on a mezzanine floor over the swimming pool changing area and the centre’s reception. The only positive to this enclosed area of the building was that the glulam beams and the associated steel had suffered less corrosion than those at poolside.

However, the negative of this area being over a mezzanine floor and tightly enclosed, meant that we did not have the option to take the loads of the roof to ground as we could at the pool side and the option of backpropping the mezzanine floor was greatly hindered by the haphazard installation of services including air handlindling, power cables, fire systems, suspended ceilings and the like. To firstly remove and then reinstate all obstructions and services  would have incurred a massive cost implication and impact on the works programme, as the gym and fitness studios are essential to the commercial side of the centres operations we were conscious of further closures to the business. 

Thinking creatively we were able to devise another way forward, we proposed the revised methodology along with structural calculations to the client’s professional team and the new approach was approved. This allowed us to work around the problem and strengthen the glulam beams without removing the existing apex connector plates and mitigated the need to provide structural propping as had been required for phase II works at poolside.

Completion/summary

In summary, the project has been a success and was completed ahead of programme and under the allocated budget, Guild handed the now safe pool and leisure centre back to the client in October 2020. The client is delighted with the end result and will be reopening the centre to the public when Government guidelines on the Covid 19 restrictions allow.

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