14

May

2021

Actavo Hire & Sales Solutions – Love cannot live where there is no truss

JOHN LAWRENCE, Design Manager, Actavo Hire & Sales Solutions, writes about how Actavo achieved the right solution in the right place at ESB’s Project Fitzwilliam.

The redevelopment of ESB’s Fitzwilliam Street site in Dublin 2 commenced in June 2017. This involved the demolition of existing buildings, the refurbishment of a number of protected Georgian structures, and the construction of a new seven-storey office block designed by internationally acclaimed Grafton Architects and O’Mahony Pike Architects.

The new seven-storey office block comprises large areas of reinforced concrete walls, columns and slabs, most of which would be considered standard construction, albeit some interesting walkway “bridges” with integrated structural handrails, large underhanging precast soffit panels, several double and triple-height decks and some very large precast façade panels. The most significant falsework challenge, however, was the Vierendeel trusses, one of which was suspended 17 metres above the basement slab. The structural arrangement for the falsework required 17m-high “towers” over a plan area of approximately 250 sq metres to support 1-sq-metre deep beams (on three floors), three floor-slabs of concrete, and all the rising elements over the three floors due to the unconventional structural arrangement. The entire weight carried by these falsework towers came to an astonishing 1070 tons, equivalent to about 148 elephants, 24 humpback whales, or three Boeing 747 jets.

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Standard methods of construction

In standard reinforced concrete multi-storey building construction (particularly flat soffit slabs), horizontal falsework systems and methods of (typical) floor by floor construction are all relatively similar. The majority of systems readily available fall into three categories;

  1. a) Loose decking (with either timber or aluminium prefabricated beams) supported on individual props,
  2. b) Proprietary panel systems fitting together in specified “grids”, or
  3. c) Fabricated “table-form”, comprising a large preassembled formwork and falsework unit forming a complete section of suspended floor slab.
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Although varying slab thickness, concrete strengths, and reinforcement percentages can affect the falsework and back-propping designs, the design principles are generally straightforward.

Alternative structural arrangements or non-standard methods of construction add additional complexities in falsework design and in method and timing of striking. These alternative methods require careful coordination between the Temporary Works Designer (TWD), the Permanent Works Designer (PWD) and the construction teams to agree on a construction sequence that;

  1. a) Achieves the integrity of the permanent structural design,
  2. b) Remains within the safe working capacity of the formwork and falsework materials (at all stages of construction), and
  3. c) Achieves practical pouring and striking times in line with the programme, and d), Provides a buildable solution for the contractor.
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Vierendeel Truss

A Vierendeel truss is a “rigid truss” structure where the structural members form rectangular openings. Standard trusses comprise members that are assumed to have pinned joints, with the implication that no moments exist at the jointed ends.

However, a Vierendeel truss comprises fixed joints capable of transferring and resisting bending moments. The use of this structural element offers several advantages in a building’s structure:

  • A large amount of the exterior envelope remains unobstructed and can be used for large windows and door openings and
  • A Vierendeel truss formed over several floors does not appear as such and will appear as slender individual slabs.

Utilising structural upstands/down-stands and columns over several internal and external courtyards, the engineering and architectural teams introduced several Vierendeel truss elements into the permanent works design of ESB’s Project Fitzwilliam.

Design Considerations

When using proprietary formwork/falsework systems to construct a standard RC concrete slab, the grid spacing required for these systems is generally dependent only on the slab thickness, prop height and prop type. In addition, in standard concrete construction, once an individual concrete slab has reached sufficient strength to be self-supporting, the falsework can be struck, ie, the load is released, allowing the slab to take its self-weight. In multi-storey construction, back-propping is used to reduce the effect of the point loads (when pouring the next floor above) on the newly poured slabs by distributing some of the wet-concrete load to the (older) slabs below. The back-propping usually is required as a sequence of point loads from falsework (during construction) acting on a newly formed slab is generally more onerous than the “in service” UDL loading.

The critical difference in designing formwork and falsework for a Vierendeel truss element is that the structural element is not fully formed until both the horizontal chords (in this case, the concrete beams, slabs, upstands and down-stands) and the vertical chords (columns) are all completed and have achieved a minimum concrete strength as specified by the PWD. This difference results in the necessity for the falsework initially used to support the lower slab to fully remain in place until the rising elements, and subsequent slabs have also been formed. Hence, this falsework must either be designed to take the cumulative construction loads until the Vierendeel truss is fully completed or must be strengthened at each stage of construction to take account of the additional wet-concrete loads for each subsequent pour.

The other consideration in the construction of a Vierendeel truss is the sequence of “striking”. Striking (as with standard construction) must be carried out from top-down to avoid “shock-loading” the lower slab. However, critically for the permanent design, when each slab is struck, the individual slabs must have sufficient strength to take their self-weight as the truss is not fully complete until the lower slab (tension chord) is fully struck. This adds further complexity into the falsework design as each stage of striking must also be carefully considered, and the falsework for each level must also be designed/ assessed for each stage as the cumulative loads from above during these stages is then concentrated through the rising elements and not distributed through all the temporary props (when the falsework for the slabs above is entirely in place).

Courtyard 2 Design

The largest Vierendeel truss structure over “Courtyard 2” comprised three floor-slabs. Hence the lowest slab formwork and falsework needed to be designed for wet concrete loads of all three floors, including all the rising elements which encompassed the entire truss structural arrangement. The subsequent slab design needed to be designed for two floors and the associated rising elements.

In addition to this complexity, the underside of the lower-most slab started at the third floor of the main structure, which was more than 17 metres above the basement floor. Each slab also included several stiffened beams on each floor, which supported a clear span of over 16 metres and were subsequently 1 metre deep. In addition to the complexity of the falsework design, the issue of assembling and dismantling these towers also required meticulous planning and several meetings with the main contractor (PJ Hegarty & Sons U.C.) the concrete subcontractor (Admore Structures Ltd) and the PWD (O’Connor Sutton Cronin) was vital at the early stages of the design.

The design principle incorporated a series of individual tower assemblies, which could be fabricated off-site and craned into place before being tied together in-situ to form a fully braced falsework structure. Each tower assembly was analysed at each stage of construction (and striking) to ensure individual prop loads did not exceed their safe working capacity. This resulted in towers being further clustered together along the perimeter to allow for the final striking phase, at which time the entire two floors above were supported by the rising elements all positioned along the same perimeter.

The off-site fabrication sped up the erection time, reduced the space needed on the site and minimised working at height. On completion of striking the falsework, the arrangement of towers allowed the individual assemblies to be separated by removing specific bracing components. The individual assemblies were then wheeled sideways from underneath the completed truss and lowered slowly sideways with a crane where they could be safely disassembled at ground level.

The falsework design was entirely carried out by Actavo’s dedicated design team and independently checked by CADD Creations.

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