MJH Engineering – Critique of the K-Brace System

Because of Wellington’s location and seismic vulnerability, the K-brace has been rightly chosen as a form of earthquake protection. The character of the K-brace system is to consume slight deflections typical of that in an earthquake. Other more expensive forms of earthquake strengthening have been shown to amplify movement of the building at particular earthquake frequencies. The nature of the site, having a reinforced concrete structure, meant that attaching the K-braces to the exterior of the building can be done with much greater ease than installing a base isolation system. The cost of developing apartments on Capital Constructions Webb st site has been significantly reduced by use of the eight K-braces installed. Ref: www.stanford.edu/~jplynch.

Some K-brace details have been surrounded with controversy. A Canadian example of a K-brace had failed at the connection of the diagonal beams to the base of the brace due to an excess in compressive forces. Particularly, the diagonal members have had trouble performing under compressive loads because of this detail seen within reference material (http://cisc-icca.ca/faq_k_brace.html). This type of detail has been successfully used in ‘X-bracing’ systems for tensile loads, however further care should and has been taken in MJH engineering’s manufacture and design of the K-brace.

Within detailing surrounding the K-brace, it is recommended a fall be put in place behind the horizontal beams. Flashings run water either out over the top of the beam, or behind the beam. Water behind the beam is expected to exit at either end of the gusset. It is feared however, that had deflection occurred in the beam and gusset, rain water will pool in behind the beam and accelerate corrosion where it will go undetected until reaching a point of failure. It is recommended that a gap either end of the gusset be opened up to allow an air passage to pass through behind the beam, therefore leaving wind to dry out the enclosed space. Another method may be to install wooden blocking behind the beam to create a fall, running out one end of the beam and gusset connection.

The existing structure, prior to development, was a reinforced concrete frame. By use of the K-brace system acting as two lines of shear walls, lateral forces typical of earthquakes can be inflicted into the building without significant damage.