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Creative use of structural form makes thin-tile vaults efficient. When vaults are designed following the thrust line, minimal material is needed for the form, which is parabolic under a discrete number of Figure 2. Rwanda Cricket Stadium Section

Figure 2 Creative use of structural form makes thin-tile vaults efficient. When vaults are designed following the thrust line, minimal material is needed for the form, which is parabolic under a discrete number of Figure 2. Rwanda Cricket Stadium Section

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Figure 3 Embedding geogrid in the edge arches uniform loads. Unreinforced masonry vaults are not typically considered suitable for seismic design. This limitation led to incorporating triaxial geogrid, in the form of Tensar TX [4], typically used for road beds, as a lightweight reinforcing material to thin-tile vaults to control dynamic lateral loads without compromising the lifespan of the structure (Ramage and Dejong [1]). The 4mm-thick polypropylene geogrid is formed of triangular elements connected at nodes (Figure 3).
Figure 3 Embedding geogrid in the edge arches uniform loads. Unreinforced masonry vaults are not typically considered suitable for seismic design. This limitation led to incorporating triaxial geogrid, in the form of Tensar TX [4], typically used for road beds, as a lightweight reinforcing material to thin-tile vaults to control dynamic lateral loads without compromising the lifespan of the structure (Ramage and Dejong [1]). The 4mm-thick polypropylene geogrid is formed of triangular elements connected at nodes (Figure 3).
Dejong et al. [5] compared empirical panel tests with an analytical model of thin-tile masonry with embedded geogrid. The panels were made and analysed using the geometry shown in Figure 4. The 650mm x 215mm x 80mm panels were tested in displacement-controlled three-point bending over a span of 520mm. After initial cracking at about 1mm of displacement, the unreinforced masonry panel had little residual capacity, whereas the reinforced panel retained some capacity due to the geogrid. The reinforced panels demonstrated roughly 80% load-carrying capacity through vertical displacement of 10-20mm, until tensile failure of individual elements in the geogrid. Figure 4 (a) Section of a three-layer geogrid-reinforced vault under a constant sagging moment. (b) Representative olan view of the forces on the geogrid reinforcing in (a). From Dejong et al. [5]
Dejong et al. [5] compared empirical panel tests with an analytical model of thin-tile masonry with embedded geogrid. The panels were made and analysed using the geometry shown in Figure 4. The 650mm x 215mm x 80mm panels were tested in displacement-controlled three-point bending over a span of 520mm. After initial cracking at about 1mm of displacement, the unreinforced masonry panel had little residual capacity, whereas the reinforced panel retained some capacity due to the geogrid. The reinforced panels demonstrated roughly 80% load-carrying capacity through vertical displacement of 10-20mm, until tensile failure of individual elements in the geogrid. Figure 4 (a) Section of a three-layer geogrid-reinforced vault under a constant sagging moment. (b) Representative olan view of the forces on the geogrid reinforcing in (a). From Dejong et al. [5]
Figure 5 : Load-displacement response of (a) the geogrid-reinforced panels, and (b) comparison to simulation results. From Dejong et al. [5] element. Combining this analysis with stress relief into adjacent elements of the geogrid due to spalling seen during the test and material properties, Dejong et al. were able to get a simulation that closely matched the experimental results (Figure 5).
Figure 5 : Load-displacement response of (a) the geogrid-reinforced panels, and (b) comparison to simulation results. From Dejong et al. [5] element. Combining this analysis with stress relief into adjacent elements of the geogrid due to spalling seen during the test and material properties, Dejong et al. were able to get a simulation that closely matched the experimental results (Figure 5).
Figure 6 The four-hinge failure mechanisms formed by the (c) unreinforced vault and (d) reinforced vault due to the impulse ground motion accelerations in (a and b). From Dejong et al. [5]
Figure 6 The four-hinge failure mechanisms formed by the (c) unreinforced vault and (d) reinforced vault due to the impulse ground motion accelerations in (a and b). From Dejong et al. [5]
The Rwanda Cricket Stadium, located in the moderate risk seismic zone of Kigali, is the first permanent building to incorporate both achievements: the use of cement-stabilized tiles made from site-excavated soil and the use of geogrid to improve its behaviour in asymmetrical and dynamic load cases. To analyse the vaults’ structure, we used graphic statics, a method to design and analyse structures based on drawn vectors and geometry (Allen and Zalewski [6]). Graphic statics’ advantage lies in the relationship between form and forces (Van Mele et al. [7]). It uses two complementary, reciprocal diagrams: a form diagram that represents the geometry of the structure and a force diagram which shows the equilibrium of the internal and external forces, resolved as the line of thrust through the structure (Block et al. [8]).
The Rwanda Cricket Stadium, located in the moderate risk seismic zone of Kigali, is the first permanent building to incorporate both achievements: the use of cement-stabilized tiles made from site-excavated soil and the use of geogrid to improve its behaviour in asymmetrical and dynamic load cases. To analyse the vaults’ structure, we used graphic statics, a method to design and analyse structures based on drawn vectors and geometry (Allen and Zalewski [6]). Graphic statics’ advantage lies in the relationship between form and forces (Van Mele et al. [7]). It uses two complementary, reciprocal diagrams: a form diagram that represents the geometry of the structure and a force diagram which shows the equilibrium of the internal and external forces, resolved as the line of thrust through the structure (Block et al. [8]).
Figure 9 Line of thrust analysis with indicative cut and earthquake load case The analysis of load cases was done with Geogebra [9] applying a 2D slicing technique, where the structure is sliced through a series of load paths turning the vault into different arches. As each vault is symmetrical, we analysed half of the structure. We sliced each half vault with 3 cuts and made a cut per each of the four cross section arches. We then analysed each cut for the three possible scenarios: static loads, wind loads and earthquake loads (Figure 9). Where the line of thrust exits the geometry, we used the perpendicular distance of deviation from the vault surface and equations (1-3, above) from Ramage and Dejong [1], modified to account for the total vault thickness, to ensure stability.
Figure 9 Line of thrust analysis with indicative cut and earthquake load case The analysis of load cases was done with Geogebra [9] applying a 2D slicing technique, where the structure is sliced through a series of load paths turning the vault into different arches. As each vault is symmetrical, we analysed half of the structure. We sliced each half vault with 3 cuts and made a cut per each of the four cross section arches. We then analysed each cut for the three possible scenarios: static loads, wind loads and earthquake loads (Figure 9). Where the line of thrust exits the geometry, we used the perpendicular distance of deviation from the vault surface and equations (1-3, above) from Ramage and Dejong [1], modified to account for the total vault thickness, to ensure stability.
Figure 10 Detail (1) and construction (r) of vault-pier interface
Figure 10 Detail (1) and construction (r) of vault-pier interface
Figure 11 Making soil cement tiles The vault construction used a mix of low-skilled and unskilled worker teams. The teams were trained by James Bellamy, an expert mason. One of the main goals of this project was to transfer the technology of thin-tile vaulting, unusual in East Africa, to local labour. There are further potential projects in the region to make use of the skills gained on this project. After the construction of the three vaults, follow- up sessions were conducted with masons and site engineers to discuss other applications and typologies.
Figure 11 Making soil cement tiles The vault construction used a mix of low-skilled and unskilled worker teams. The teams were trained by James Bellamy, an expert mason. One of the main goals of this project was to transfer the technology of thin-tile vaulting, unusual in East Africa, to local labour. There are further potential projects in the region to make use of the skills gained on this project. After the construction of the three vaults, follow- up sessions were conducted with masons and site engineers to discuss other applications and typologies.
The second phase was building the guidework so the newly trained masons were able to visualize the shape of the vaults. Full formwork was made for the construction of the edge arches, with triangular pieces that describe the curved intersection between the vault surfaces and the arches. In another departure from tradition, because the arches would be in contact with rain water, the first layer of tiles was laid with white cement instead of fast-setting gypsum mortar as the latter is soluble in water. The guides and formwork were cut and assembled by local carpenters (Figure 12).
The second phase was building the guidework so the newly trained masons were able to visualize the shape of the vaults. Full formwork was made for the construction of the edge arches, with triangular pieces that describe the curved intersection between the vault surfaces and the arches. In another departure from tradition, because the arches would be in contact with rain water, the first layer of tiles was laid with white cement instead of fast-setting gypsum mortar as the latter is soluble in water. The guides and formwork were cut and assembled by local carpenters (Figure 12).
Figure 13 Rwanda Cricket Stadium © Jonathan Gregson; used with permission
Figure 13 Rwanda Cricket Stadium © Jonathan Gregson; used with permission
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