Design flaw leads to crack in new Kirkland bus ramp

A new express bus ramp above Interstate 405 in Kirkland has cracked because of a design blunder, so it will need to be partly rebuilt at a cost of $2 million to $3 million.

The project at Totem Lake will not be finished until summer, four to six months late, according to the state Department of Transportation (DOT), which disclosed the problem in a news release Thursday.

The crack was discovered in December while contractors were building bus stops as part of a four-lane access ramp for high-occupancy vehicles in the median, just north of the Northeast 128th Street overpass.

The roadway cracked lengthwise where a wide concrete slab hangs partly over the freeway in what the DOT says is an innovative cantilevered design.

Like an overloaded teeter-totter, the slab cracked down the center and sagged an inch at one of its far ends, said John Conrad, the DOT's assistant secretary for engineering and regional operations.

Simply put, the DOT's engineering team misunderstood how the materials in the 75-foot-wide slab would perform.

There was not enough reinforcing steel inside the concrete, Conrad said.

To fix things, he said workers will insert tight steel bands to compress the concrete, a method known as "post-tensioning," which should make the slab more rigid.

The $58 million interchange project, funded mainly by Sound Transit, is one of many places where high-occupancy lanes exit directly from freeways to park-and-ride lots. However, Conrad said the other new HOV ramps don't have the same flawed design.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com.