Tectonus DMAX is an alternative to a BRB - but how do they differ?
When it comes to braced frame buildings in seismic areas the leading contender has long been the traditional Buckling Restrained Brace (BRB). More recently however Tectonus has launched a direct alternative in the DMAX brace. This article delves into the comparative advantages and disadvantages of DMAX, focusing on performance, cost, compliance, and constructability.
BRBs and DMAX are both part of the lateral stiffness solution and provide damping at the same time. Compared to conventional braced frames, both systems can bring advantages in the following areas:
- Reducing seismic design loads on the structure and foundations, creating construction cost savings
- Protecting the structure from damage and downtime in the event of an earthquake.
In comparing the two systems however, some key differences become evident.
Buckling Restrained Braces
BRBs have been a staple in seismic design since they were introduced from Japan in the late 90s. Since then, several leading suppliers have emerged and BRBs have become codified around the world. In a BRB the steel core resists the load, while the concrete-filled outer casing restrains it from buckling, ensuring that the brace yields in a controlled manner under both tension and compression. This behavior provides reliable energy dissipation during seismic events.
Image credit: AISC
Key Features of BRBs
Cost Effective: Being of a relatively simple design compared to some other devices like viscous dampers, BRBs have become a good choice for the value conscious. There are many suppliers, and the cost of production has been optimized over the years.
Controlled Yielding: BRBs are designed to yield in a controlled manner, providing predictable energy dissipation and enhancing the overall seismic performance of the structure.
High Ductility Factor: BRBs are at the top end of ductility allowable in the US building code (R=8). A higher ductility factor allows for lower loads on the rest of the structure and foundations.
Post Earthquake Damage Limitation: BRBs are designed to yield in an earthquake, concentrating damage in the brace’s yielding core. Rather than inspect and repair the whole structure, only the BRB needs to be replaced.
Relatively Straightforward Compliance. BRBs have been around for decades which has led to their codification in US and Canada. Engineers can use prescribed R values and permitting is typically straightforward. Connection detailing is also well understood.
DMAX Braces
The DMAX brace is a cutting-edge seismic braced frame designed as a direct alternative to BRBs. It incorporates Tectonus’ proprietary friction damper technology, known for its high damping capacity and stable performance across various seismic events. Recent advances have led to the optimization of production cost so that a DMAX brace is comparable in price to a BRB of a similar capacity.
Like a BRB a DMAX brace is supplied end to end as a replacement for conventional braces
The DMAX brace system has been installed in projects in New Zealand and is currently undergoing testing per OSHPD/HCAI in the US for use in a hospital retrofit project.
Key Features of DMAX
High Ductility Factor: With its reliable Elastic Perfectly Plastic (E-P-P) hysteresis behavior, DMAX effectively dissipates seismic energy, reducing the forces transmitted to the structure. Like BRBs a ductility factor of R=8 can easily be achieved, reducing loads on the structure and foundations.
Lower Overstrength Factor: DMAX boasts an overstrength factor of 1.15 which is significantly lower than for BRBs (1.6 to 2.5). This lower overstrength factor translates to further reductions in seismic demand on the structure, allowing for even lighter structural members, connections and foundations.
Tuneable Capacity: the capacity of DMAX is tuneable to the engineer’s target capacity to within +/-1 kip which means that no additional material or design tolerance is needed above and beyond the exact requirement.
Ease of Installation: DMAX is mostly a hollow section making it lighter weight to install. Length adjustment can be done by shimming. This makes pin connections easily achievable whereas for BRBs the tolerances are often to tight to be practical.
No Post-Earthquake Repairs: Unlike BRBs, which are likely to require inspection and replacement after a significant seismic event, DMAX has no yielding parts so there is nothing to repair or replace after an earthquake.
Individually Tested. Every single DMAX damping unit is tested to the design level earthquake before it is shipped to site. This provides ultimate assurance to the engineer that all of the devices will perform as expected. Since they yield, BRBs cannot be tested in this way.Comparative Analysis
Performance
Although the ductility and damping provided is similar, the hysteresis curves of a BRB (left) and DMAX (right) show the differences in behavior. The relatively flatness and consistency of the DMAX curve explains its lower overstrength factor.
Typical hysteresis curves of BRB (left) and RSFJ (right).
Cost Efficiency
DMAX’s combination of lower overstrength, high damping, and tuneable capacity allows for substantial cost savings compared to BRBs. The reduction in structural actions, allows engineers to design for smaller member sizes, less foundation works, and reduced connection detailing. Cost savings are typically in the form or materials, connections, and labor. The reduction in materials also translates into a design with lower embodied carbon.
Post Earthquake Damage and Downtime
Both systems are designed to concentrate energy dissipation within the brace itself. BRBs are designed to yield, requiring replacement of the core after a significant event. In contrast, DMAX is designed to perform consistently without needing post-earthquake repairs. This feature reduces costs associated with repairs and minimizes downtime and disruption to building operations.
Constructability
DMAX is designed to be a straightforward replacement for BRBs in braced frame buildings. Its installation process is similar to that of BRBs, making it an easy upgrade for projects looking to enhance seismic performance without significant changes to the design or construction process. However, DMAX is comparatively lighter than a BRB since it does not have a concrete-filled core, and this can lead to some efficiencies during the construction phase. In addition the ability to adjust the length of DMAX by shimming makes pin connections easily achievable.
Compliance
Being new to the market DMAX is not yet codified which typically means an alternative approval pathway – in the US commonly referred to as ‘Performance Based Design’. The additional cost of alternative design and peer review is typically more than offset by the savings DMAX produces through high damping and low overstrength.
In Conclusion
For structural engineers, the choice between DMAX and BRBs hinges on several factors, including total construction cost, post-quake performance, and compliance. DMAX offers a compelling alternative to traditional BRBs, with its lower overstrength factor, high damping capacity, and damage-free design providing significant advantages. By reducing the seismic demand on structures, DMAX enables cost savings in construction while enhancing the overall resilience and performance of buildings in seismic regions. BRB is more straightforward when it comes to compliance but if there is time and client appetite, then DMAX can be a compelling option.
In summary, DMAX represents a forward-thinking solution in seismic engineering, aligning with the industry’s goals of improving safety, reducing costs, and minimizing environmental impact. As structural engineers continue to seek innovative ways to design and construct resilient buildings, DMAX will become a valuable tool in their arsenal.
Ready to learn more? Please go to Tectonus DMAX webpage.
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For a comparison between BRBs and Tectonus RSFJs check out this article.