00203 lisa oliver
TRANSCRIPT
MOTAT AVIATION DISPLAY HALL – TIMBER ENGINEERING
CASE STUDY
Lisa Oliver1, Cameron Rodger
2, Stephen Clarke
3
ABSTRACT: A new Aviation Display Hall was completed in May 2011 at the Auckland Museum of Transport and
Technology (MOTAT). The building is predominantly one large open space (like an aircraft hanger) but with the fit-out
and finishes completed up to museum standards. This paper outlines the structural systems used in the building,
including how a clear span of 42 m was achieved that allows aircraft to be suspended from it. The LVL member
fabrication, connection details and the assembly process are described and a brief comparison of how this building
compares to what could have been constructed using steel is also presented.
KEYWORDS: MOTAT, Museum, LVL, Box sections, Steel gusset plate, portal frame, LVL cross-bracing
1 INTRODUCTION 123
A new Aviation Display Hall was completed in May
2011 at the Auckland Museum of Transport and
Technology (MOTAT). The building is predominantly
one large open space (like an aircraft hanger) but with
the fit-out and finishes completed up to museum
standards. The building is constructed out of laminated
veneer lumber (LVL) and is 55 m long, 50 m wide and
15 m high. It will be used to house some of the aircraft in
MOTAT’s collection, including both a Solent and a
Sunderland (the giant 22 tonne flying boats).
The architects of this project were Studio Pacific
Architecture from Wellington; together with the
MOTAT Board they made the decision early on to
investigate building the new display hall in timber.
Holmes Consulting, as the structural engineers on the
project were also interested in pursuing a timber option.
Carter Holt Harvey (CHH) Woodproducts became
involved early in the design phase, letting the MOTAT
Board tour their own LVL buildings so that they could
get a feel for what the finished building might be like
and providing technical, material and construction
1 Lisa Oliver, Holmes Consulting Group, Unit five, 295
Blenheim Road, Christchurch, New Zealand, Email:
[email protected] 2 Cameron Rodger, Carter Holt Harvey Woodproducts New
Zealand, 173 Captain Springs Road, Onehunga, Auckland,
New Zealand, Email: [email protected] 3 Stephen Clarke, NZ Strong Construction, 108 Mt Eden Road,
Mt Eden, Auckland City, New Zealand, Email:
methodology advice. NZ Strong joined the team as the
construction contractor.
Now complete, the building has opened as an integral
part of the museum complex and has picked up several
architecture, sustainability and engineering awards from
the New Zealand Institute of Architecture and in the NZ
Timber Design Awards. The interior of the finished
building with some of the display planes is shown in
Figure 1.
Figure 1: Exhibits in the new MOTAT Aviation Display Hall (Patrick Reynolds)
2 STRUCTURE
In this section the building form is described along with
how LVL was used and detailed throughout the
structure.
2.1 BUILDING FORM
The primary structure is a series of portal frames. These
support the roof structure and provide the lateral
resisting system in the north-south direction. Cross
bracing is used to form a roof diaphragm and to resist
horizontal loads along the building in the east-west
direction. The portals frames have double columns and a
clear span of 42 m. The structural ridge line is off centre
and the south column slopes outwards to create visual
interest. The portal frames, purlins (secondary roof
members), girts (wall members) and wall cross-bracing
are all made from LVL. Figure 2 shows the portal frames
during construction.
Figure 2: MOTAT Aviation Display Hall during construction
Supported by the double portal columns on the south
side of the building is a mezzanine level. This is also
almost entirely constructed from LVL members. The
extensive use of timber is shown in Figure 3.
Figure 3: Interior of the MOTAT Aviation Display Hall showing south wall and mezzanine (Patrick Reynolds)
The new building is primarily one large open space
which will allow flexibility to the display of aircraft. As
well as the majority of the building’s structure being
timber, so are many of the internal linings. The architects
used timber to bring warmth and character to the
enormous volume.
The building site was historically used as a landfill and
the refuse is still decomposing. Therefore the whole
building is supported by steel piles which found on the
basalt rock below the landfill. The main floor slab was
cast in a single concrete pour and is post tensioned to
prevent excessive shrinkage cracking. The polished floor
slab is exposed and has been designed to support the
220 kN Solent aircraft anywhere on it.
2.2 LAMINATED VENEER LUMBER (LVL)
As discussed above, the majority of the structural
elements are made from LVL. This is an engineered
wood product manufactured from timber which is rotary
peeled, dried and laminated together in continuous long
lengths. Typically, the veneers are laminated together
with all the timber grain orientated in the same direction.
For the MOTAT project hySPAN (a LVL product
produced by CHH Woodproducts) was predominantly
used in the primary structure, with lower stiffness
hyCHORD used for secondary framing such as purlins
where cost advantages exist. These products are
manufactured at Marsden Point, New Zealand, from
sustainably grown New Zealand plantation pine forests.
The use of LVL members over other wood products was
chosen due to its uniformity, consistent structural
performance and ability to come in long lengths with
large cross-section dimensions. Out of the LVL products
that CHH Woodproducts supply, hySPAN was chosen as
it has a high modulus of elasticity (E = 13.2 GPa) and
reliable bending stress.
For this project custom runs were completed. This
allowed high visual grade veneers to be selected for the
exposed LVL members, non standard length, width and
thickness members to be used, and for the addition of
cross-brands where required.
As the LVL members are exposed in the finished
museum, it was important that the finish on the LVL was
aesthetically pleasing. This required special selection of
the exterior veneers and care taken not to get excessive
resin on the exposed surfaces.
The largest pieces of LVL used in the project was over
18 m long, 1.2 m wide and 63 mm thick, these were used
as webs in the fabricated box beams. Some members
were also required to have cross bands; the reason for
this is described in more detail in the next section.
2.3 BOX BEAM PORTAL FRAMES
The portal frames are constructed from LVL box beams
as the long span, large loads and deflection criteria
meant it was not efficient to use solid LVL rafters.
The clear span between the inside portal frame columns
is 42 m, this is currently the largest clear span for a
timber portal frame building in New Zealand. A cross-
section through the building is shown in Figure 4.
Figure 4: Structural section through the MOTAT Aviation Display Hall
The building is used to display aircraft, and some of
these will be suspended from the box beam rafters. The
design of the portal frames had to allow for this loading,
including flexibility as to how many planes would be
suspended and where they could be hung from to allow
exhibitions to be rearranged in the future.
As well as the exhibition load, the building had to be
designed for high wind pressures. At the west end of the
building is a large roller door. This can be seen in Figure
5. The four leaves slide right open to allow aircraft to be
moved in and out of the building. Under wind loading
the door is considered a dominant opening and this
increases the potential wind pressures that the building is
designed for. The wind load cases governed the design
over the seismic load cases.
Figure 5: MOTAT Aviation Display Hall viewed from the north-west (Patrick Reynolds)
The rafters are 1200 mm deep and 426 mm wide, a
typical rafter cross-section is illustrated in Figure 6. The
box beams were glued and nailed together off site by an
experienced fabricator using a total of 300 litres of glue
and 560 000 nails [1]. The nails were designed to
transfer the shear flow between the member’s webs and
flanges. The number of nails along the rafters was able
to be varied depending on the shear flow demand.
Figure 6: Cross-section through typical LVL box beam rafter
When designing the box sections there were numerous
issues to consider, including potential cupping of the
web members which was mitigated by using
cross-bands. LVL is typically manufactured with the
grain of all veneers running along the length of the
member. However, in this instance the box beam web
members were very slender (1200 deep x 63 wide) and
this made them prone to cupping when there was a
differential in moisture content between the air inside the
box beam and that outside it. To give the webs some
transverse strength a single “cross band” was added
close to each face of the LVL member. In these veneers
the orientation of the grain is 90 degrees to the length of
the member giving it stability and also reducing the
likelihood of nails or screws splitting the timber. The
cross-band locations are indicated in Figure 7.
Figure 7: Typical web section of LVL with cross-bands indicated
2.4 PORTAL FRAME CONNECTIONS
The portal frames were designed with pinned base
connections and fixed knee (column to rafter) and apex
connections. To allow transportation of the portal frame
elements to site each portal frame was split into eight
components; four column and four rafter segments.
Splitting the rafter into four pieces resulted in three
moment resisting connections along the member, one at
the apex and two approximately at points of
contraflexure (locations of low flexural demand).
The moment connections in the portal frames were one
of the most interesting aspects of the design and took
numerous iterations to optimise. Two types of moment
connections were used, these were; “screw rings” with
external steel gusset plates at the knees and apex, and
“nail rings” for hidden timber splice connections at the
points of contraflexure.
The steel gusset connections are shown in Figure 8. The
choice to use steel gussets was determined by the
magnitude of the forces being transferred over the
connections. The gusset plates were pre drilled with the
screw ring pattern and painted prior to installation. The
use of screws rather than nails was made necessary by
the use of steel gusset plates. The type of screws used
were 14 gauge, type 17 self drilling hex head screws.
This allowed them to be power driven without the need
for pre-drilling. It is estimated that 57 000 such screws
were used in the project [1].
Figure 8: Steel gusset moment connections on site during construction
The design procedure for designing nail and screw rings
is well published; however, panel shear across the
connection was found to govern the nail and screw
layout, which is not something commonly checked.
For the main column to rafter connections, the number of
screws rings calculated to meet the required moment
demand was three; however, the shear this would induce
between the sides of the “ring” was larger than the shear
capacity of the web of the LVL box section. Therefore,
the distribution of screws was altered so that the sides of
the “ring” perpendicular to the grain are two screws wide
and the sides parallel to the timber grain are four screws
wide. This can be seen in Figure 9 which is the structural
drawing of a typical rafter to column connection.
Figure 9: Steel gusset moment connection structural drawing
A typical nail ring connection is shown in Figure 10,
these had six nail rings and the design was governed by
the forces in them during the rafters being lifted into
place. To create the “hidden” connection the “gusset” is
actually another small section of box beam within the
box section.
Figure 10: Nail ring moment connection during construction
2.5 OTHER LVL COMPONENTS
It was not just the portal frames that were fabricated
from LVL, but also the purlins (secondary roof
members), girts (wall members), mezzanine floor joists
and even the wall cross-bracing. The cross-bracing can
be seen in Figure 11.
Figure 11: Timber cross-bracing on south wall of the MOTAT Aviation Display Hall (Patrick Reynolds)
To create the wall cross-bracing, double timber elements
were used as tension braces down the sides of the
building with steel flitch plate type connections. Steel
plates were fixed between the two pieces of timber at the
connection locations using a combination of steel dowels
and bolts. A larger steel bolt was then used as a pin to
connect the cross-braces to the rest of the structure.
Typical brace connections are shown in Figure 12 and
Figure 13.
Figure 12: Structural drawing of a typical cross-brace connection
Figure 13: LVL cross-bracing end connection on site during construction
3 CONSTRUCTION
Each LVL portal was transported to site in 8 pieces (the
longest segment was 18 m).
The nature of MOTAT, and other LVL based portal
frame systems, allows for purlins fixed into the side of
the portal frame rafters on the ground in bay multiples
and lifted into place as shown in Figure 2 and Figure 14.
Four rafter components were spliced on the ground with
purlins framed into the side of the rafters creating a
laterally stable system for lifting. This enabled
secondary framing to be installed on the ground limiting
the amount of time in scissor lifts, dramatically
increasing the productivity on site and reducing
occupational health and safety risks. In the case of
MOTAT some efficiencies were also introduced during
the erection process by allowing the installation of the
ceiling framing support members on the ground instead
of at height.
Figure 14: Erection of the LVL portal frames
An erection sequence was developed based on lifting
two complete bays. Two 200t cranes were employed to
lift the 54 tonne load into the air so it could be connected
to the columns by the steel gusset plates. The load was
lifted into position within fifteen minutes but it took
around 5 hours to screw the fixings into the columns.
The columns were stood the day before the lifting of the
rafters. Two, double bay lifts were completed with end
bays stick framed, creating only three infill bays.
Figure 15: Portal Rafters being screwed to the columns
The same sequence was followed until all of the portals
were erected. Upon their completion the LVL perimeter
wall girts were installed using elevated working
platforms. The entire LVL structure was completed
within 40 working days.
A time laps recording of the construction can be found
on the MOTAT website [2]
4 COMPARISON WITH STEEL
The choice to build from timber has created an
aesthetically pleasing, distinctive building that is
creating a lot of interest, but it has also been a cost
effective and environmentally sustainable solution.
During the design phase a comparison was completed
between a LVL portal frame and an equivalent steel
portal frame building [3]. The comparison looked at
price and sustainability. For simplification the
comparison was not completed on the MOTAT project
as architecturally the form of the building would have
changed somewhat if the material had been different.
Instead a theoretical 1800 m2 warehouse was designed;
both the steel building and LVL building were designed
using the same design parameters for the same location
(Auckland, New Zealand).
Pricing of the two schemes was completed
independently. The price included material supply,
detailing and creation of workshop drawings, fabrication,
supply to site and erection. The result indicated that the
LVL building was approximately 9 % cheaper.
To assess the relative sustainability of the two options
Life Cycle Assessments were completed by SCION. The
assessment boundaries included cradle to factory gate
and cradle to grave.
It was assumed that at the end of the buildings life, the
LVL building would go to landfill and the steel system
would be recycled (re-melted and re-used).
The key measurement parameter in the assessment was
Global Warming Potential (GWP). The assessment
showed that for the Cradle to Grave scenario the LVL
building had a 42 % lower GWP.
5 CONCLUSIONS
The new MOTAT aviation display hall pushed the limits
of a typical timber portal frame and turned a construction
type that is often associated with industrial and
warehouse type buildings into a building of museum
quality. It has also shown that timber can be used to
create attractive, cost effective and environmentally
beneficial structures on a commercial scale. It is a
project we are all proud to have been a part of.
ACKNOWLEDGEMENT
We would like to acknowledge the Museum of Transport
and Technology Board for choosing to build with timber
and the other members of the design team for their
contribution to the project.
REFERENCES
[1] Carters your building partners: MOTAT Aviation
Display Hall – A Carters LVL project. Carters
Auckland, 2011.
[2] Museum of Transport and Technology, Aviation
Display Hall, Part 2: The New Display Hall,
http://www.motat.org.nz/explore/exhibitions/part-2-
the-new-display-hall, accessed 6 February 2012.
[3] CHH Woodproducts Portal Suite, Information
Bulletin: LVL Portal Frame vs Steel Portal Frame,
http://www.chhwoodproducts.co.nz/portalsuite/,
June 2009.