Striatus – a first of its kind 3D concrete printed arched bridge – now open

Striatus is an arched masonry footbridge composed of
3D-printed concrete blocks assembled without mortar or reinforcement. The 16 x
12 metre footbridge is the first of its kind, combining traditional techniques
of master builders with advanced computational design, engineering and robotic
manufacturing technologies.

Exhibited at the Giardini della Marinaressa during the Venice Architecture
Biennale until November 2021, Striatus has been developed by the Block Research
Group (BRG) at ETH Zurich and Zaha Hadid Architects Computation and Design
Group (ZHACODE), in collaboration with incremental3D (in3D) and made possible
by Holcim.

Proposing a new language for concrete that is structurally informed,
fabrication aware, ecologically responsible and precisely placed to build more
with less, Striatus optimises the properties of masonry structures, 3D concrete
printing (3DCP) and contemporary design; presenting an alternative to
traditional concrete construction.

The name “Striatus” reflects its structural logic and fabrication process.
Concrete is precisely printed in layers orthogonal to the main structural
forces to create a “striated” compression-only structure that requires no
mortar or reinforcement. 

Using a special concrete ink
developed by Holcim, this method of 3D concrete printing combines the
principles of traditional vaulted construction with digital concrete
fabrication to use material only where it is structurally necessary and eliminate waste.

As the construction does not need mortar, the blocks can be dismantled, and the
bridge reassembled at different location. If the construction is no longer
needed, the materials can simply be separated and recycled.

Striatus Bridge at the European
Cultural Centre’s ‘Time Space Existance’ exhibition, Giardini della
Marinaressa, Venice, Italy

Image & video download

Striatus website

Strength
through geometry

Striatus is an unreinforced concrete structure that achieves strength through
geometry. Concrete can be considered an artificial stone that performs best in
compression. In arched and vaulted structures, material can be placed precisely
so that forces can travel to the supports in pure compression. Strength is
created through geometry, rather than an inefficient accumulation of materials
as in conventional concrete beams and flat floor slabs. This presents
opportunities to significantly reduce the amount of material needed to span
space as well as the possibility to build with lower-strength, less-polluting
alternatives.

Striatus’ bifurcating deck geometry responds
to its site conditions. The funicular shape of its structural arches has been
defined by limit analysis techniques and equilibrium methods, such as thrust
network analysis, originally developed for the structural assessment of
historic masonry vaults; its crescent profile encompasses the thrust lines that
trace compressive forces through the structure for all loading cases.

Steel tension ties absorb the horizontal
thrust of the arches. Neoprene pads placed in between the dry-assembled blocks
avoid stress concentrations and control the friction properties of the
interfaces, echoing the use of lead sheets or soft mortar in historical masonry
construction.

In plan, the boundaries of the structure form
deep arches that transfer horizontal loads (for example, from visitors leaning
against the balustrades) to the supports in pure compression. Advanced discrete
element modelling (DEM) was used to refine and optimise the blocks’ stereotomy
and to check stability of the entire assembly under extreme loading cases or
differential settlements of the supports.

The bridge’s 53 3DCP voussoirs have been produced
using non-parallel print layers that are orthogonal to the dominant flow of
forces. This avoids delamination between the print layers as they are held
together in compression. The additive manufacturing process ensures the
structural depth of the components can be achieved without producing blocks
with a solid section, hence reducing the amount of material needed compared to
subtractive fabrication methods or casting.

Striatus follows masonry structural logic on
two levels. As a whole, the bridge behaves as a series of leaning unreinforced
voussoir arches, with discretisations orthogonal to the dominant flow of
compressive forces, following the same structural principles as arched Roman
bridges in stone. Locally, on the level of the voussoir, the 3DCP layers behave
as traditional brick masonry evident in the inclined rows of bricks within
Nubian or Mexican vaulting.

Circular
by design

Circular by design, Striatus places material only where needed, significantly
reducing its environmental footprint. Built without reinforcement and using dry
assembly without binders, Striatus can be installed, dismantled, reassembled
and repurposed repeatedly; demonstrating how the three R’s of sustainability
(Reduce, Reuse, Recycle) can be applied to concrete structures.

Reduce: 
– Lowering embodied emissions through structural geometry and additive
manufacturing that minimises the consumption of resources and eliminates
construction waste.
– Placing concrete only there where needed, 3DCP minimises the amount of material
required, while the low-stress, compression-only funicular geometry of Striatus
proposes the further development of 3DCP that will enable the use of much
lower-strength, less-polluting printable materials.
– Compared to embedded reinforcement in concrete, Striatus uses external ties
to absorb the thrust of its arched shape and dramatically reduce the amount of
steel required. A high carbon-intense material, steel reinforcement (100%
recycled) per unit mass is more than ten times that of a standard concrete.

Reuse: 
– Improving circularity and longevity. Unlike conventional reinforced concrete
structures, Striatus is designed to be dry assembled without any binder or
glue, enabling the bridge to be dismantled and reused in other locations. Its
funicular design ensures the 3DCP blocks experience low stresses throughout
their use, resulting in no loss of structural integrity. Striatus separates
components in compression and tension, ensuring external ties can be easily
accessed and maintained, resulting in a longer lifespan for the entire
structure.

Recycle:  
– By ensuring different materials are separated and separable, each component
of Striatus can easily be recycled with minimal energy and cost. 3D printing
also avoids the waste and costs associated with single-use moulds.
Additionally, the component materials within Striatus remain separate and
separable with the use of mechanical connections such as simple dry contacts
between the voussoirs rather than chemical glues or binders, ensuring a simple,
low-energy recycling process at the end of the elements’ life, potentially
after multiple cycles of reuse.

Robotic 3D
concrete printing

Unlike typical extrusion 3D printing in simple horizontal layers, Striatus uses
a two-component (2K) concrete ink with corresponding printing head and pumping
arrangement to precisely print non-uniform and non-parallel layers via a
6-axis, multi-DOF robotic arm. This new generation of 3D concrete printing in
combination with the arched masonry design allows the resulting components to
be used structurally without any reinforcement or post-tensioning.

To prevent misalignment between the direction
of structural forces and the orientation of material layers that arises from
typical shape-agnostic slicing of explicitly modelled geometry, a
custom-developed design pipeline was formulated for Striatus to ensure that its
printed layers are wholly aligned with the direction of compression forces
throughout the entire bridge and also locally through each 3D-printed block. To
address issues and challenges that could prevent in-between stability during
printing, the coherence and feasibility of the gradually evolving print paths
have been modelled using a Functional Representation (FRep) process.

This process encodes and continuously checks rules of minimum overlap, maximum
cantilever between print layers and print length, print speed and the volume of
wet concrete extruded. These measures, typically used in horizontally layered
3DCP, have been advanced and refined to work on an inclined-plane setting:

– The angular differences between start and
end planes of all 53 printed blocks have been simultaneously adjusted to meet
multiple criteria such as an appropriate structural contact and angle between
adjacent blocks, and maximum print inclination.
– The careful design and iterative refinement of the hollow cross sections and
infill triangulation have ensured that material is placed corresponding to the
precisely analysed, local structural performance of each block. This design and
optimisation has been applied to each individual layer of every block (with 500
print layers on average per block), ensuring that all blocks are as hollow and
light as possible, and consequently use the least amount of material possible,
while maintaining structural integrity under all loading conditions.
– The resulting intricate cross-sectional design has been processed into a
single, continuous print path meeting various criteria that include appropriate
print speed and turning radii, structurally required material width and
thickness, and controlled expression of naturally occurring printing artefacts.

A nuanced aspect of robotic 3DCP masonry is the re-introduction of intelligence
and highly skilled labour into the manufacturing and construction industry. The
digitisation of fabrication and digital augmentation of skilled assembly and
construction techniques makes historically-accrued knowledge accessible to
younger generations and enables its systematic upgrade towards industrialised
construction through the use of computational and robotic technologies. In
stark contrast to a brute force, and often materially wasteful economy biased
towards automation and assembly line production, 3DCP masonry introduces
possibilities of a symbiotic human-machine economy. This promises an
environmentally, socio-culturally and economically sustainable alternative to
its 20th-century predecessor.

Computational
design-to-construction integration

Integrating design, engineering, fabrication and construction, Striatus
redefines conventional interdisciplinary relations. The precise manufacturing
of the blocks was enabled by well-defined data exchange between the various
domain-specific software toolchains involved in the process. This
co-development approach was facilitated through the use of COMPAS, an
open-source computational framework for collaboration and research in the AEC
industry, which enabled the fluent interaction among the key players of the
project, working together in five different countries, under a very tight
schedule and budget, at a time in which travelling was not possible.

Disruptive outlook
Striatus offers a blueprint for building more with less. Created with the same
structural principles and a similar fully-integrated computational
design-to-fabrication approach that form the basis of the vaulted,
rib-stiffened, unreinforced concrete floors being developed by the Block
Research Group in partnership with Holcim, Striatus proposes an alternative to
the standard inefficient floor slabs within any building.

Compared to typical reinforced-concrete flat floor slabs, this new floor system uses only 30% of the volume of concrete and just 10% of the amount of steel. The very low stresses within the funicular structure also enable the use of low-embodied-carbon concrete that incorporates high percentages of recycled construction waste. Prefabricated and dry-assembled, and therefore fully demountable and reusable, this floor system is easily and cleanly recyclable at end-of-life. With an estimated 300 billion square metres of floor area to be constructed worldwide over the next 30 years, and floors comprising more than 40% of the weight of most high-rise buildings (10+ storeys), introducing the principles demonstrated by Striatus would truly disrupt the construction industry — transforming how we design and construct our built environment to address the defining challenges of our era.

Photograph by Naaro

Source: zaha hadid

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