Thin Shell Concrete Roof (HyPar)
This past summer Fifth Wind was honoured to host George Nez, who at 92 is still travelling all over the world promoting his Thin Shell Concrete Roof HyPar designs. Several people attended the work shop and were inspired by him. His technique offers a wonderful alternative to asphalt shingles. The latex modified concrete shell is an inexpensive, more environmentally sound and extremely durable product which also allows for a reduction on the amount of wood used in traditional roof construction. Fifth Wind intends to bring more public attention to this great alternative.
See below for technical information and also check out George on YouTube:
HyPar Roof OverviewAcrylic cement composite can be applied to an arched Hypar (hyperbolic paraboloid) shaped cloth stretched across a 4-sided pyramid framework of wood or bamboo to create a hypar thin shell concrete roof that is far superior roofing alternative to corrugated metal and other roofs.
Even with a final thickness of approximately 1 cm – a TSC Hypar roof is strong enough for multiple people to walk on.
Hypar structures have been built for decades but the most closely observed demo structure was built in Boulder in 1996 – enduring Colorado freezes, thaws, winds and up to 3 feet of snow – and is in near perfect condition after 14 years. In warmer climates, these roofs should last many decades.
TSC hypar roofs can be retrofitted onto existing walls but the ideal building method is “roof first” with lumber or bamboo frames. Non-load bearing walls and light foundation and flooring can then be added afterwards underneath to bring great savings to the overall building costs.
Note that because multiple TSC layers each require a day or two to cure, 7 days minimum is required to create each roof. Thus this method strongly lends itself to multi-roof projects so that teams can rotate around roofs in varying stages of completion for maximum time efficiency. Note also that a significant amount of space for staging can be required for production in a medium or large scale project.
Durability of TSC Hypar RoofTSC Global is fortunate to have a demo structure built by Engineers Without Borders in Boulder in 1996. After 14 years this structure has been subjected to a full range of climate stresses including 3 foot snow loads, extreme heat and cold, 80+ mph winds and intense high altitude UV exposure. Visual inspection by the TSC Global and Colorado School of Mines engineering team revealed almost no structural deterioration at all. In fact the hypar surface was in perfect condition, with hairline fractures having developed only along the flat areas on the ridge beams.
More importantly, the 1 cm. thick TSC roof surface displayed flexibility and resilience when walked on, and our technician was able to bounce the roof membrane which showed the flexing response for the entire structure. Panos Kiousis was quite impressed and visual observation indicates how resilient the roof would be in the event of even a large earthquake.
An important advantage to TSC Hypar buildings is the minimal to zero need for lumber. Once completed, the concrete is “doing all the work” instead of the original pyramid frame. Thus TSC Hypar roofs have been successfully built using 3 inch green bamboo. The cost-savings and environmental value of using fast-growing bamboo instead of lumber are quite apparent.
Because the bamboo beams cannot transfer significant loads through nails and screws due to longitudinal tear weakness, George Nez has developed an ingenious method whereby joints are prepped using nails, wires, and sometimes metallic brackets, and then wrapped with strong canvas dipped in latex modified mortar. The wrapping naturally shrinks and hardens during curing and creates a quite strong connection.
The hypar shape can create many roof forms, but the two currently promoted by TSC Global are Hat shape (see Nuba guesthouses below, et al) and Cross Gable (see Birambye kitchen above). Both create efficient coverage- always perfect squares.
TSC Global/Nuba Water Project Guesthouse Juba, South Sudanhttp://www.nubawaterproject.org/
Hypar roofs thus can then be configured modularly and the edge junctures between roofs can be easily and thoroughly sealed for an infinite variety of potential floor plans.
Roof venting for air circulation and light
The top vent and cap is an important and simple modification. Particularly in very hot climates, the top venting allows hot air escape creating a much cooler more comfortable interior. Adding a ceiling and sealing off the attic space creates further cooling. Venting of cook smoke is also an important health feature. A vent panel made of translucent material such as corrugated plastic can let in diffused light – particularly of use for schools and health clinics – and still offer full rain protection.
Along with the individual roof-by-roof venting, an additional benefit is neighborhood air movement. A presumably urban high density of TSC Hypar roofs, by virtue of the swooping hypar shape, will create convections cells in the air spaces above, even on still days. The drafts would make heavily populated neighborhoods cooler, more comfortable, less odorous and healthier.
Note that the loft space in each structure is a significant amount of useable space. Light floor rafters creates a large storage space, and heavier loft floor rafters would create useable bedroom spaces.
Earthquake Resistance ModificationsA TSC roof – particularly with embedded chicken wire mesh and due to the roof’s very light mass – demonstrates resilience to failure or collapse. It’s very light weight would dramatically decrease chances of injury or worse in earthquake scenarios. Even in the most severe earthquake, TSC roofs themselves would likely exhibit little more than lateral cracking.
Recent work in overall building design by Colorado School of Mines Structural Engineering professor Panos Kiousis has suggested that simple wall-embedded cross-braced panels secured to a strong ring beam, with adequate fastening of roofs to posts, should create an earthquake resistant building, still at low costs. Model and shake-table tests pending – application made for National Science Foundation NEES grant.
Excerpts from Colorado School of Mines National Science Foundation NEES grant:Vision
The vision of the proposed project is to apply experimental and analytical techniques to evaluate and optimize the seismic design and performance of a structural system consisting of a thin shell roof made of latex-modified concrete and its supporting frames. This structural system may be readily constructed to provide inexpensive long-term shelters in impoverished regions where modern construction equipment and materials are not immediately accessible.
Earthquakes and their after effects serve as a constant reminder of the perils of inadequate structural designs. Recent events such as the Haiti earthquake and the subsequent aftershocks, however, remind us of the more significant problem that inadequate designs are often the result of poverty and the accompanying short term improvisations for temporary shelters and rebuilds rather than a lack of knowledge in earthquake resistant design. Advanced engineering as applied to Los Angeles, Taipei, or Tokyo clearly demonstrate that we have made significant progress in developing small and large structures that can withstand significant earthquakes. On the other hand, regions like Haiti demonstrate equally clearly that we have not addressed sufficiently the issue of earthquake resistant structures that are built in impoverished regions with limited resources. This is of significant concern, as several areas with high seismic risks are also areas with low Human Development Indices (HDI)1 (Fig. 1), particularly in Southeast Asia, East Africa, Central and South America, and the Caribbean Islands.
Countless examples around the world demonstrate that similarly sized earthquakes may cause significant loss of life and property in impoverished regions, while their effects in more economically advantaged regions are significantly less [e.g., Bulut et al.2005, EEFIT 2007, EEFIT 2008, EERI 2005, Glass et al. 1997, Jones et al. 1993]. It becomes therefore important to address more earnestly the aspects, processes, and financing of seismic infrastructure engineering for impoverished regions. In this proposal, we aim to address this issue based on an existent structural design, specifically developed for humanitarian relief purposes, that already has a reasonably large number of applications in regions around the world. The techniques to construct this structure have been developed and implemented around the world by Denver-based engineers Dr. George Nez and Dr. Albert Knott as well as a Denver based organization, TSC Global. All of the above have offered their expert knowledge to assist the PIs on this project.
This roof structure has never been tested in the laboratory or in real life under appreciable earthquake loads, although it has been used in areas of high seismic risk. It will be shown that the dynamic characteristics of the roof and the overall structural system change significantly with the supporting conditions, even though their static response to gravity loads is relatively unaffected. The goal of this study is therefore to perform analytical and experimental seismic analyses of this structural system aiming to identify its dynamic characteristics and evaluate its ability to withstand significant earthquake loads. The earthquake loads will be obtained from the regions of the intended use of this structural system. At the outcome, necessary improvements for seismic performance will be proposed, and detailed guidelines will be provided addressing realistic building practices for the impoverished regions for which this structural system is intended.
Static Stress Distribution of TSC Hypar Roof
As demonstrated in the plan view stress contours of a 5m x 5m roof with a height of 2.5 m in the middle and thickness of 10 mm, the self-weight of such structure produces very small tensile and compressive stresses. There are not sufficient experimental data describing the compressive and tensile strength of this latex modified mortar. However, assuming a rather conservative estimate of fc’ = 10MPa(1450 psi), this structure works under its own weight with a factor of safety of 20 in compression and a factor of safety of 15 in bending cracking based on modulus of rupture [ACI-318M-08]. The specific problem requires an additional vertical pressure of 3 kPa (63psf) to achieve tensile cracking, while operating with a factor of safety of 1.5 in compression, indicating sufficient strength to high-power wind loads.