DESIGN GUIDANCE FOR REDUCING EMBODIED CARBON IN STRUCTURAL SYSTEMS

Serviceability Criteria

Serviceability criteria, including deflection and vibration requirements, can often control the structural design, particularly for longer-span systems.  On this basis, it is important to evaluate the criteria being used for the design, and identify whether the anticipated structural performance (often established via computer models) is realistic.

Take advantage of higher deflection limits for different structural elements as allowed in the building code, e.g. for roof structures or areas without sensitive partitions.  Evaluate atypical deflection limits e.g. for facade or elevator systems, and ensure that such limits are fully understood before applying these to the structural design.

When establishing vibration criteria, evaluate the specific requirements and space use.  Communicate the impact of these criteria on the structural design to the project team, and evaluate any assumptions being made in analysis models, such as connection or support fixity, which should be as indicative as possible of ‘real’ performance.  Note that standard bolted connections which are assumed to be pinned for linear static design can typically be assumed as being fixed for a vibration analysis, in accordance with AISC design guidance.

Where only localized areas of specific vibration performance are required, consider locating these elements in the naturally stiffer areas of the bay, for example near columns, walls, or perimeter spandrels.

For specialty structures, consider dampening as an alternative to stiffening of structural members as a means for controlling vibration effects.  In some specialized cases, a performance-based design approach may present an alternate pathway to standard code-applied serviceability limits, such that a more carbon-efficient design may be realized.

Maximize Design Utilization

Studies indicate that structural engineers will typically design to maximum utilization ratios of approximately 0.8, with average utilizations of approximately 0.6.  Assuming a linear relationship between utilization and material use, this suggests that structural engineers are designing with significantly more material than necessary.

The application of maximum utilization ratios below 1.0 should be avoided during final design. In particular, be aware of the effect of compounding safety factors and rationalization methods (such as enveloping member demands instead of designing individual members), which may lead to some elements being particularly underutilized.

Consider also, the application of material strengths on a project in relation to embodied carbon.  Higher strength may enable significant material savings for heavily loaded members, close to their strength design limit.  However, lower strength materials may be more appropriate in cases where serviceability requirements control the design.  This tradeoff varies by structural element type and should be evaluated on a case by case basis early in design and as the design progresses.

References:

Rationalisation versus optimisation – getting the balance right in changing times (Source: The Institution of Structural Engineers)

Structural Optimization

Where possible, structural optimization should be used to reduce and minimize the amount of structural material and/or embodied carbon in the project. A variety of methods exist, ranging in accessibility and complexity. The three classical types of structural optimization, which typically target material efficiency, include: size optimization, shape optimization, and topology optimization as described below.

1. Size optimization: at the most granular scale, individual framing members (beams or columns) can be optimized to be the most efficient (lightest) section based on design criteria set by the engineer.  Many analysis and design programs offer a version of this, via an “optimize” or “auto-select” option within the program. A user can set design criteria holistically for the project, for an area of the building, or individually for specific members. This set of criteria can include minimum/maximum section depth, deflection criteria, vibration performance, maximum utilization ratio, and more.
2. Shape optimization: structural components may be shaped to use material where it is most needed. This method can be effective for optimizing the depth of bending elements, such as shaped beams and vaulted floor slabs. Recent research has demonstrated affordability and scalability in shaped concrete floor systems (see References).
3. Topology optimization: this type of optimization can be applied at various structural scales (e.g. trusses, beams and floors, wall layout, lateral systems, joint connections). This method determines the topology, or layout, of a minimum-volume structure under given loading, support, and serviceability conditions. The layout may be across a continuum design domain (e.g. for walls, floors, or beams) or a discrete design domain (e.g. for trusses or shear wall layout). Aside from topology, other design parameters such as the size and material of the structure can also be determined from the optimization result; it can thus be effective in earlier stages of design. Software is available to perform topology optimization, such as those from Altair, Dassault Systemes, and Autodesk, Comsol, and Ansys. Even without access to such tools, the ground structure method can be used to manually explore discrete topologies (e.g. selecting a truss topology) for material efficiency. Complex results typically present a challenge for fabrication, but practical results are possible (see References).

Tradeoffs between custom optimized designs and standardized designs may also need to be considered (i.e. the lower-carbon custom solutions may require more time and money to design and/or construct). Anticipating these tradeoffs before a project begins can help enable the future feasibility of lower-carbon design solutions. It’s never too late to start or enhance streamlined optimization workflows.

References:

Minimizing embodied energy of reinforced concrete floor systems in developing countries through shape optimization (Source: Engineering Structures Journal)

Design, fabrication and testing of a prototype, thin-vaulted, unreinforced concrete floor (Source: Engineering Structures Journal)

Connecting architecture and engineering through structural topology optimization (Source: Engineering Structures Journal)

Truss topology optimization of timber–steel structures for reduced embodied carbon design (Source: Engineering Structures Journal)

Refocusing Modern Methods of Construction on the Climate Emergency (Source: IStructE)

Rationalisation Versus Optimisation–Getting the Balance Right in Changing Times (Source: IStructE)

Design for Resiliency

Designing for intense natural disasters, such as seismic events, can lead to high embodied carbon and embodied energy emissions when the required post-disaster renovations are considered. Current code-based design is intended to prevent structural collapse, but at the cost of extensive damage to the building, mainly including non-structural elements.  By optimizing a structure to perform in a more resilient way, engineers can mitigate the embodied impacts of any future disasters.  Using alternative design methods such as performance based design (PBD) to design a more resilient structure can proactively prevent these emissions (and all related costs as well). However the designer must also weigh the pros and cons of this added embodied carbon upfront, versus adding carbon emissions during a renovation in the future. 

Reference:

Seismic Performance Assessment of Buildings (Source: Carbon Leadership Forum) 

Application of Typical Details

The use of typical details on design drawings should also be evaluated in relation to embodied carbon. Typical details should be refined on a project basis, such that they are representative of loading and design conditions that will be encountered.  The application of typical details to atypical conditions is considered a poor engineering practice that can lead to excessive material use and waste and greater embodied carbon for the structure.

Design for Adaptability and Deconstruction (DfAD)

Buildings are often demolished not because they are worn out, but because they no longer are suitable for their purpose. In some situations, it can even cost less to tear down and reconstruct than to renovate. Designing for adaptability can reduce the future economic incentive to demolish a building when needs change. Designing for deconstruction and reuse increases the chances that materials and components are extracted and incorporated into new construction when the building reaches its end-of-life.

Strategies for adaptability include:

• Open plans that permit flexibility of space usage.
• Structural systems that have reserve strength and stiffness to accommodate different uses, or that can be readily upgraded to carry more load.

Strategies for both adaptability and deconstruction include:

• Simple, transparent structural systems that can be observed and evaluated.
• Independent systems that allow, for example, MEP systems to be upgraded without disturbing the structural system.
• Durable systems.
• Labels on components with strength characteristics (similar to grade stamps on wood framing).
• Clear explanation of design criteria (i.e. loading, deflection criteria, vibration criteria) on the general notes.
• Dry connections and mechanical fasteners, such as bolts and screws, in place of welds and adhesives.

Designers must balance DfAD with design optimization goals. A fully optimized structure designed for light loads may prohibit future adaptation to a use requiring heavier loads. Designing for adaptability and deconstruction will more effectively address long-term climate goals than short-term goals. 

References:

Designing Structural Systems for Deconstruction: How to Extend a New Building’s Useful Life and Prevent it from Going to Waste When the End Finally Comes 

Future Adaptability (Source: American Institute of Steel Construction)

Deconstructable Systems for Sustainable Design of Steel and Composite Structures (Source: American Institute of Steel Construction)

Use High Strength Material

For hot-rolled sections, Grade A992 became the standard grade in 1998 and represented a 40% increase in the strength of structural sections from 36 ksi to 50 ksi. Today, A913 Grade 65 (65 ksi) is produced domestically and is particularly appropriate for large columns and belt trusses. Reduction in material used due to higher strengths has a linear impact on embodied carbon reduction. 

References:

Who makes the shapes you need? (Source: American Institute of Steel Construction)

Design Load-Optimized Members

The entire length of a steel beam or a column is not necessarily experiencing the same level of stress. At times a smaller section containing less material can be used if a plate is welded to the beam or column to handle the higher level of stress in a given region of the member. This option should be evaluated carefully in order to balance the effects of reduced member weight and environmental impacts with the additional fabrication labor time and cost.

References:

On Demand Course – Design of Reinforcement for Steel Members: Part 1, Part 2 (Source: American Institute of Steel Construction)

Castellated Beams

Creative fabrication practices can add substantial value to a traditional steel beam. For example, a wide flange beam may be split into two halves, separated, staggered, and welded together to form a castellated beam. The new beam weighs approximately the same but is 40% stronger and 50% deeper than the original beam. This approach provides a substantial functional increase with very little added embodied carbon, particularly when considering that less than 10% of the embodied carbon of a fabricated structural steel member comes from fabrication. 

References:

Design Guide 31: Castellated and Cellular Beam Design (video: NASCC 2018 Session) (Source: American Institute of Steel Construction)

Cambering

Floor members can be cambered mechanically, or by heating forces, to deflect the beam in the opposite direction of the deflection from the loading. In some cases, weight and embodied carbon savings as much as 25% can be accomplished. Cambering beams of less than 24 feet in length and cambering of less than ½ inch is not recommended. The use of camber should be discussed with the fabricator and the cost and time tradeoffs should be weighed by the design team. 

References:

Design Guide 36 – Design Considerations for Camber (videos: NASCC 2012 Session, NASCC 2021 Session) (Source: American Institute of Steel Construction)

Address Fireproofing

Steel projects often call for cementitious spray fireproofing, which contributes additional embodied carbon, so strategies to reduce and/or replace its use are warranted.

The level of required fireproofing is generally defined according to the construction type classification as defined by IBC chapter 6.  Architects can consider pushing for the lowest possible construction type to limit the amount of fireproofing required.  They can also limit the shaft locations to fewer bays, for construction types where the shaft openings are driving the extent of fireproofing.

Alternative fireproofing methods to cementitious spray fireproofing are also available, such as intumescent paint and mineral wool board fireproofing.  These materials have lower associated embodied carbon emissions, however their use should be weighed against potential cost increases..

As an alternative to structural fire resistance ratings, ASCE 7 sanctions the use of performance-based structural fire design. Notably, ASCE 7-22 Table 1.3-5 now provides applicable reliability targets for structural fire design. This methodology permits synergy between the structural and fireproofing designs/specifications, which can significantly reduce reliance on fireproofing (e.g., allowance for non-uniform application of fireproofing). This is demonstrated in the ASCE Structural Fire Design Guide, which is freely available at the link provided below.  Note that the application of performance-based structural fire design typically requires consultation with a building code official and a peer review.

References:

Performance-Based Structural Fire Design (ASCE-SEI)

Concrete Strength and Mix Selection

The majority of embodied carbon emissions in concrete are a direct result from the cement production process, meaning concrete with higher levels of cement will generally have higher associated embodied carbon emissions.  The use of blended cements and supplementary cementitious materials (SCMs) can be advantageous for reducing the required cement content, as well as improving workability and durability performance. Refer to the SE2050 specification guidance for additional information related to procurement strategies for reducing embodied carbon in concrete.

Over-specifying concrete strength results in increased embodied carbon. Specify only the strength that is required for the design of members. If durability provisions in the code require a higher strength, consider taking advantage of that in design.

The use of lightweight concrete in the design of concrete structures should also be approached with caution.  Lightweight concrete mixes can have significantly higher embodied carbon emissions versus normal-weight mixes due to the emissions associated with lightweight aggregates, despite also reducing floor plate dead loads on structure.

Industry-wide (average) EPD and regional benchmarks for concrete can be found on the NRMCA website, which are organized by concrete strength, SCM content and type of aggregate.

NRMCA: Environmental Product Declarations (Source: NRMCA)

Cast-in-Place Concrete

Simple concrete flat slabs can be relatively efficient for low span dimensions and have historically been an attractive option for buildings due to their associated ease and speed of construction. However, for longer span dimensions or higher loading conditions, it is generally advantageous to utilize formwork systems that will increase structural depth and stiffness for a given volume of concrete, such as ribbed slab / joist slab and waffle slab systems.  Voided slabs, which use void-formers cast into the concrete slabs, can also be an effective way of increasing the stiffness to weight ratio and reducing embodied carbon of the structural deck.

Post-tensioning (PT) of concrete slabs can also be an effective means for reducing structural depth and associated embodied carbon.  During PT construction, a pre-compression is applied to the slab after it has been cast, via a series of tendons and anchorages.  When evaluating post-tensioned concrete slab systems, consideration should be given to early strength requirements and the impact of a higher cement content on the overall embodied carbon emissions.

References:

Comparison of embodied carbon in concrete structural systems (Source: MPA The Concrete Centre)

Comparing different strategies of minimising embodied carbon in concrete floors: post-tensioning embodied carbon savings increase with span (Source: Journal of Cleaner Production)

Precast Concrete

With precast concrete construction, the prefabrication of structural floor panels can eliminate the need for temporary formwork systems and reduce waste.  Similar to cast-in-place concrete, the use of profiled slabs can be an effective way of increasing the stiffness-to-weight ratio and reducing embodied carbon. An advantage of profiled precast concrete slabs is that they can be constructed without custom temporary formwork systems meaning they are generally much faster to construct than equivalent cast-in-place systems.

Prestressing of precast slabs is also an effective means for reducing structural depth and embodied carbon.  Prestressed slabs are typically constructed using high strength prestressing strands cast into floor planks.  Using hollow-core planks, with longitudinal voids at the middle of the slab, can also serve to increase the stiffness-to-weight ratio and reduce embodied carbon in precast slabs.  Similarly, when evaluating prestressed concrete slab systems, consideration should be given to early strength requirements and the impact of a higher cement content on the overall embodied carbon emissions.

Reinforcing

Steel reinforcing provides an important contribution to the total embodied carbon of reinforced concrete structures and should not be overlooked.  In applications controlled by sectional strength, the use of high strength reinforcement can be advantageous for reducing the amount of reinforcement required and total embodied carbon.  Doing so can also help to address reinforcement congestion and improve constructability.

The contribution of lap splices and bends to the total reinforcing quantity should also not be overlooked when evaluating the embodied carbon in reinforced concrete structures.  The use of mechanical coupling devices can be an effective method for reducing some of this material, which can also help to reduce reinforcement congestion and improve ease of construction.

Concrete Masonry Units (CMU)

When designing with CMU, the use of allowable stress and strength design methods should be prioritized over empirical methods or rules of thumb, to eliminate overdesign.  Consideration for material strength can also help to reduce material quantities and embodied carbon. Increasing the specified compressive strength (f’m) may result in smaller reinforcing bars, larger spacing of rebar, reduced wall thickness, and reduced lap splice requirements. Consideration for the prism test method instead of the unit strength method to determine compressive strength can also be advantageous for an efficient design.

Consideration for standard block dimensions is also important for reducing waste in CMU construction.  Where possible, masonry wall lengths, openings and reinforcement spacing should follow a standard 8-inch block module to reduce cutting of units and construction waste. BIM software for masonry can assist with design and layout of walls noting these inefficiencies.

Advanced Framing (Optimum Value Engineering)

Advanced Framing, also known as Optimum Value Engineering, is a lean design approach to stick-built wood framing. These practices include: framing at 24 inches on center, aligning studs with joists and rafters to enable use of single top plates, aligning openings with stud spacing, eliminating or reducing header sizes at non-bearing walls, and eliminating unnecessary framing at wall intersections.

References:

U.S. Department of Energy Advanced House Framing

Energy Start Advanced Wall Framing.

Engineered Wood

While engineered wood products such as Laminated Veneer Lumber (LVL) and wood I-Joists are strong and stiff relative to dimension lumber, the embodied carbon of these products is much higher per pound of material due to the glues and manufacturing processes. Compare the embodied carbon of systems framed with engineered wood and with dimensional lumber in cases where either material could be used.

Mass Timber

The typical application of mass timber is cross-laminated timber (CLT) decks supported by glue-laminated (glulam) beams and columns. Column grids should be optimized to maximize CLT deck spans while minimizing beam spans. Column transfers and excessive cantilevers should be avoided as these may require unreasonable member sizes or the introduction of additional framing.

Early participation in the design process by a timber fabricator will allow for member and connection optimization based on the fabricator’s particular experience and capabilities. Coordination with mechanical distribution systems is often a critical step in optimizing the system design.

Alternatives to concrete topping, such as raised floor systems or gypcrete, should be considered to reduce embodied carbon depending on fire rating requirements, sound attenuation, and architectural acceptability.

References:

Mass Timber Design Manual, Vol. 2 (Source: WoodWorks)

Design Materials to Work Together 

Floor sheathing adds strength and stiffness to floor joists which may allow designers to reduce joist sizes. The shear flow between the sheathing and the joists must be checked and the fasteners designed to resist the shear force. Wood I-joist manufacturers frequently publish span values that account for this composite behavior. Consider taking advantage of this extra strength in all joist designs.

Aiming for Material Efficiency Regardless of Sequestration

Bio-based materials have the ability to sequester a portion of a tree’s biogenic carbon for temporary periods of time, but this does not mean that buildings should use higher quantities of timber. Rather, the overarching approach of using less material still applies. 

References:

Timber and carbon sequestration (Source: The Institution of Structural Engineers)

Custom Sections

The digital manufacturing of mass timber elements offers an opportunity to produce custom structural elements that achieve material efficiency. Some examples of simple customized geometries include arched and tapered elements.

References:

Richmond Olympic Oval (Source: naturally:wood)