Structural Technologies Aiding Green Building
Greener Concrete
Greener Concrete
Currently a lot of concrete products that are not green still have some fly ash content in them. By increasing the fly ash content to 20 to 70% of the cement mixture we are reducing the amount of fly ash that goes to landfills. Fly ash is a byproduct of burning coal for energy, it use to be sent up into the sky before environmental regulations finally stopped or reduced the fly ash into the atmosphere. As a result there is a lot of fly ash in the chimneys coal fire plants. (The best way to reduce fly ash is to reduce coal fire plants.) If a 50% fly ash mixture where used widely, it would reduce the worlds CO2 production by 4% (that is big).
By adding fly ash to the concrete mix we are creating a concrete that works differently than regular concrete, it does not make it worse, and it is less expensive than cement. It takes longer to set and dry- so the engineer needs to create a time line for the working strength of the concrete so the job can continue while the concrete is still curing. It takes less water, requires different admixtures, and can have more shrinkage cracks.
Material Reduction
By using fewer materials we can reduce the environmental impact. But there will be more labor involved in carefully calculating each beam individually, as opposed to the current standard of consistent and repetitive beam sizes. This could also reduce the quantity of bolts, welds, reduce shear wall length, and change stud spacing to 24 or 32 on center.
By use of a higher strength concrete, material can be reduced by thinner slabs, shorter and thinner shear walls, reduce deflection, and ultimately building weight.
Using high strength steel is not very practical. Standard steel shapes usually use standard steel strengths. It would be better to reduce the steel weight by increasing the depth of the beams for less deflection and widths of the posts for more efficient strengths.
Less material can reduce construction cost. There will be a learning curve for labor to be more attentive to small and frequent detail changes. More detailed plans will require more construction management and coordination. Wood projects or advance framing techniques will probably benefit the most from these changes. Concrete would benefit the least.
Advanced Systems
Advanced systems like energy dissipaters, system isolators, buckling restrained braced frames and steel plate shear walls can reduce materials and effects of earthquake damage on buildings. These systems require additional engineering and cost.
To justify these costs we need to consider the Life Cycle the building? How long is the building to last? Would it be better to spend more on materials and have a longer building life? Can the building adapt to other uses? And what will be the impact on the building when the earthquake hits tear it down or replace some isolators?
Green Aspects of Structural Materials
Current LEED credits for engineering are gained in these areas:
Fly ash counts as pre-consumer recycled content, MR4.
Significant use of fly ash has been awarded Innovation in Design credit, ID1.
Crushing existing concrete for aggregate counts as post consumer recycled content credit, MR4.
Using special aggregates, pozzolans and add mixtures can help achieve Innovation in Design credit, ID1.
Showing the extended life of the building may also help achieve Innovation in Design credit, ID1.
LEED does not look at the life cycle, flexibility or reuse of a building.
Recycled wood can go into particleboard, mulch, fuel and firewood. Larger members can be reused, but smaller members like 2x4s will split apart when nailed. Windows and doors can be recovered and reused. Wood is easily deconstructed and reused in some manner. Wood is the greenest building material. Trees store carbon. Wood comes from a rapidly renewable resource. The creation of lumber uses relatively low amounts of energy sometimes their own sawdust (biofuel) creates the energy to mill the timber.
Steel is the most recycled material in the world. Over 80% of the steel comes from recycled product. Reusing steel beams is fully practical. Steel can be constructed with deconstruction in mind by using less weld and more bolts. Avoid unusual or custom shapes that are more difficult to reuse. Steel is the strongest per volume building material used. Steel manufacturing has been dramatically cleaned up, most hazardous waste associated with manufacturing is being recovered and used beneficially. Steel is produced more efficiently yet still takes a lot of energy to melt.
Concrete can be reused as base material, gravel, aggregate for new concrete, and the steel can be recycled. Concrete slabs and walls cannot be recycled for structural use. Concrete has the largest carbon footprint. The production and pouring of one ton of concrete creates 1.25 tons of CO2 and significant heat. One 90-pound bag of cement creates 22291 cubic feet of CO2 (a volume equivalent to 28 cubic feet).
Using an existing building can be greener then building a new green building. When we consider all the waste from demolition, existing embodied energy in materials, labor, transportation and time in an existing building we can save more energy through reuse.
Reusing a building has social, economic and cultural impacts. Adding life, strength and safety to an old building often has less impact on the environment. Building design and structural systems should allow for easy reconfigurations and different uses. The envelope or curtain wall system should be removable to allow future renovated or modernized exterior. Simple building shapes would save in renovation costs.
by: John Classick
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