Leisure
20 NSC
May 20
18 sizes on each floor, most of which require
transfer structures to take into account the
changing column lines.
The most significant parts of the steel
frame form the main theatre, which is a
triple-height space on the ground floor. A
total of four Vierendeel trusses span the
performance area and create the columnfree
space. The trusses are 11m-long ×
4.9m-deep and each weigh 11t when fullyassembled.
As well as forming the roof the main 200-
seat theatre, the trusses create a gantry for
maintenance walkways and scenery storage
within their depth, while also supporting
the floor above.
“The trusses are working very hard, they
support third floor precast planks that sit on
the top chord as well as supporting a line of
columns that extend up to roof level, which
are positioned roughly at mid span,” says
Mr Kealy.
Space was at a premium during
construction, and bringing the trusses to
site was a challenge as there was not enough
room for them to be delivered in one piece.
“The trusses had to be brought to
site piece-small, assembled within the
theatre’s footprint and then erected using
a 250t-capacity mobile crane,” says Mifflin
Construction Contracts Director Dave
Ornsby.
The main theatre is an acousticallytreated
box-in-box, with a double row
of columns and acoustic pads on all
connections, which will prevent any noise
escaping or getting in.
A similar acoustic treatment approach
has been taken with the structure’s second
and smaller theatre space, which is also
located on the ground floor.
With no requirement for a gantry, the
single storey-high second theatre has been
formed with a series of 8.5m-long UB
sections that span and support the ceiling.
Adjacent to both theatre spaces on the
ground floor, the main double-height foyer
is another column-free space formed with a
series of 8.7m-long UB sections.
Highlighting the complexity of the
structure’s steel frame, there are two
double-height rehearsal spaces sitting
at second floor, adjacent to the main
theatre’s gantry level.
The fourth floor accommodates openplan
offices and features the project’s longest
spans. Again, a series of UBs form the
12m-long spans of these spaces, while also
supporting columns on the uppermost floor
that divide meeting rooms from a kitchen
and a production studio.
For the installation of precast elements
and the steelwork erection, Mifflin
Construction primarily used the site’s tower
crane. The exceptions to this were the areas
around the tower crane, where 60t-capacity
and 95t-capacity mobile cranes had to be
used, as minimum radius could not be
achieved by the tower crane itself, as it was
too close.
The Ovalhouse is scheduled to open its
doors in spring 2021.
The primary reason for choosing a Vierendeel truss is the opportunity
to use the space within the depth of the truss – at the Ovalhouse,
the space is used for all the essential paraphernalia which must be
located above the performance area. The term “truss” is hardly appropriate
as a Vierendeel truss is a continuous frame, with rigid joints at the nodes.
The bending moments within the frame increase towards the supports, so
Vierendeel trusses are often characterised by vertical members which increase
in size from the centre of the span to the supports, to accommodate the
larger moments. It is generally not convenient to change the depth of the
chords, so these are usually the same size throughout.
The joints between the chords and vertical members will be modelled as
‘rigid’ in the numerical analysis and it is important the real details deliver this
assumed behaviour. The joints must obviously be strong enough, but must
also be sufficiently stiff to behave as rigid joints. Often, joints in Vierendeel
trusses are fully welded, but this was not possible at the Ovalhouse as the
truss had to be assembled on site. The haunches shown in the photograph
give greater resistance for the bolted connections between the most heavily
loaded vertical members and the chords.
Vierendeel trusses are often one of the possible solutions to questions set
in the Institution of Structural Engineers examination – so knowing how to
determine an approximate bending moment diagram could be a useful skill.
A simple approach is to assume a point of zero moment at the mid-point
of every member. A worked example of this method can be found in older
versions of the Steel Designer’s Manual.
A simple approach to the analysis has particularly relevance for a Vierendeel
truss – named after Arthur Vierendeel, a Belgian Civil Engineer and Professor.
He is credited with the following opinion: “When constructing in metal the
dimensions must be determined a priori by aesthetic considerations and
only afterwards should mathematical formulas be used”. One wonders what
Professor Vierendeel’s view would be of the modern dependence on software
to analyse the form of structure that carries his name.
Advisory desk articles 293 and 294 discuss the joints in welded Vierendeel
trusses with particular reference to the web panel zone within the joint.
Vierendeel trusses
David Brown of the SCI offers some comments on the
design of Vierendeel trusses.
Steelwork was chosen
as it provided the most
efficient solution for
the theatre's complex
design.
/Leisure_buildings#Theatres_and_auditoria
/Trusses
/Floor_systems#Precast_units
/Construction
/Construction#Mobile_cranes
/Acoustics
/Steel_construction_products#Standard_open_sections
/Construction#Steel_erection
/Construction#Tower_cranes
/Trusses#Vierendeel_trusses
/Continuous_frames
/Construction#Site_bolting
/Advisory_Desk_Notes
/Welding