Performance of Retrofitted Wood Diaphragms

Jazalyn Dukes and Stephanie German , Ph. D. Candidates, Julie Dykas, M.S. Candidate, and Roberto T. Leon, Professor

School of Civil and Environmental EngineeringGeorgia Tech, Atlanta, GA

Collaborators:Jason Ingham, Professor, University of Auckland

Lisa Moon, Ph.D. candidate, U. of Adelaide, Australia David Biggs, Consultant, Troy, New York

Background: • Damage to unreinforced masonry (URMs) structures

continues to be one of the greatest sources of human and economic losses loss after earthquakes. • As a very large portion of the building stock in seismic areas is

made up of URMs and many have historical significance, techniques for their retrofit and retrofit performance are of great interest to structural engineers. • An common principle in URM retrofits is the idea of

improving diaphragm action so that all the resistance of all the building elements can be activated. • Because of the very stiff, brittle and heavy nature of URM

elements and the light and flexible nature of diaphragms (mostly light wooden floors), achieving effective diaphragm action is difficult.

The photo above shows a typical roof diaphragm retrofit where the anchors appear not to have worked because of a mismatch in materials, poor installation, or insufficient design strength.

Objective: To reevaluate current USA strengthening practices based on the Christchurch experience, and propose changes to USA URM strengthening and retrofitting techniques.

Approach: Use a series of 6 to 8 case studies to:• Assess, qualitatively, effects of construction practices and

materials.• Document typical details and retrofits (anchorages,

horizontal bracing, out-of-plane reinforcement).• Document typical approaches to retrofitting (code issues

and calculations).• Document performance and lessons learned.

Field Work: Two students spent one month collecting data on anchor performance (*), diaphragm behavior (**), obtaining retrofit plans and photographing damage.(*) Dr. A. Schultz, U. of Minnesota(**) Ms. Lisa Moon, U. of Adelaide

Current Work: Original building and retrofit plans for 8 building are being examined and compared to observed performance . Report expected May 2012.

Preliminary Conclusions:• The number and location of anchors needs to be reconsidered;

probably many more anchors than previously thought are needed.• Careful thinking about load paths and load magnitudes in

diaphragms needs to be an integral part of the design; partial retrofits did not seem to perform well.

• The cyclic performance of interface zone in post-installed anchorages into weak mortars and brick was the key to performance; much of this is related to quality control and quality assurance.

• Lesson learned are valid for USA practice.

Feb. 25, 2010

April 15, 2010

Successful Example: Although a large majority of the partial retrofits did not work well, there were some very successful examples such as 69 Colombo Street (Malthouse), the oldest malthouse in NZ which had been converted to a children’s theater by a private foundation. The strengthening scheme consisted primarily of replacing damaged wood elements in the diaphragms and the introduction of a large steel hat truss on the roof to tie the structure together and transfer the lateral loads to a new shear wall (J. Hare, SE, Holmes Consulting, Christchurch).

(a) View of exterior(b) Strengthened 1st floor roof(c) Strengthened 2nd floor roof(d) Force introduction to shear wall(e) Floor anchorage to new wall(f) Roof anchorage to new wall

(a)

(b)

(c)

(d)(e)(f)