BLOG: Seismically upgrading low-rise wood frame structures

0 57 Technology

by JOC News Services

Lee Rowley, the principal for Nanaimo, B.C. based Herold Engineering Limited, headed up the "challenges and opportunities of seismically upgrading low-rise wood frame structures" session at the Wood Design and Construction Solutions conference on Feb. 28 in Vancouver.
BLOG: Seismically upgrading low-rise wood frame structures

Rowley is involved in the province of British Columbia's school seismic upgrade program, and said low rise wood frame structures are considered to perform well in an earthquake, and upgrading them is typically cost effective. They are usually in good condition, but they have common deficiencies.

But if they are not maintained properly, "they can go downhill pretty quickly," Rowley said, and mechanical and electrical systems upgrades can vary tremendously.

If well maintained, he said, building envelope issues are typically energy efficiency related, rather than to water ingress.

Common issues are the load path, the roof, the walls and the foundation.

Lack of connectivity between floors and poor load paths down to foundations or between the diaphragm and roof are common concerns, he said.

Roof issues include horizontal boards having low capacity, along with a lack of drag strips or other ways to collect the load, he said.

Wall issues are similar, with low capacity and inadequate nailing and blocking causing seismic weakness. Large window openings are also an issue in older buildings.

Shallow foundations, especially on rockier ground, need to be upgraded, Rowley said.  Poor soil is not common on Vancouver Island, though it is a problem in the Lower Mainland.

A lack of perpendicular foundation walls can weaken a building and increase the likelihood of seismic damage.

Solutions include changes to code, roof and floor diaphragm solutions, wall and foundation upgrading and non-structural work.

Municipalities generally limit restrictions on further upgrades, he said, because they often opt to do seismic upgrades. That isn't a guarantee though, he stressed.

Upgrading a roof typically involves adding plywood, Rowley said, but generally buildings already have wood based roofs.

Tying a seismic upgrade to upcoming renovations is a smart path to take, Rowley said, whether it involves the roof, floor, or mechanical/electrical upgrades.

Upgrades are also weather dependent, he said, and in the winter working from the ground u makes more sense, while in summer a top down approach is optimal.

Wall upgrading should be ideally done from outside, and on smaller buildings try to maximize the use of the diaphragm, he said.

"Work on the outside of the building and you may not have to go in it," Rowley said. On bigger buildings, working inside is somewhat unavoidable, he added.

Washroom can be a problematic area due to wet walls, and storerooms can be inaccessible.

"Find out what you have access to and work around what you don't," he said.

Foundation upgrades can mean working past a wide range of problems, but for normal soils and conditions "we typically dig trenches around a foundation," Rowley said.

Considerations also have to be made as to the effects of excavation on paving and landscaping as well as access to crawlspaces.

"Strategic thinking is important. Consider upgrading longer lengths of wall to keep tension forces down and you have a higher chance of reusing the foundation," he said.

Non-structural solutions are being recognized as cost effective and are of growing importance to any upgrade strategy, Rowley said.

Mechanical and electrical systems should be considered in an upgrade, he said, and are usually tied to the roof.

Opportunities during an upgrade include performing a concurrent energy upgrade, reducing weight and phasing. Phasing involves incorporating smaller elements to avoid having to perform a full upgrade.

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