1

Presentation for the

AIA – San Mateo

County Meetingon the

CUREE-CaltechWoodframe Project

29 September 2004

Acknowledgements

• FUNDING: Provided primarily by the Federal Emergency Management Agency (FEMA) under the Hazard Mitigation Program, Section 404 of the Robert T. Stafford Act (Public Law 93-288 as revised), through a California Governor’s Office of Emergency Services (OES) Hazard Mitigation Grant award.

2

Consortium of Universities for Researchin Earthquake Engineering (CUREE)

Project DirectorBob Reitherman

A non-profit corporation dedicated to the advancement of earthquake engineering research, education, and implementation

Acknowledgements

OUTLINE

• Introduction• Building Configuration: Open Front• Building Configuration: Soft/Weak Story• Building Configuration: Hillside Homes• Force Distribution and Glued Floors• Detailing for Improved Shear Walls

3

Introduction…

Woodframe Project Origin and Goals

Casualties: 24 of the 25 fatalities in the Northridge Earthquake that were caused by building damage occurred in woodframe buildings (1)

(1) EQE and Calif. OES, 1995

S Property Loss: Half or more of the $40 billion in property damage was due to damage to wood buildings; approximately. $15 billion in insured loss (2)

(2) Charles Kircher et al., 1997, and Robert Reitherman, 1998

Functionality: 48,000 housing units, almost all of them in woodframe buildings, were rendered uninhabitable by the earthquake (3)

(3) Jeanne B. Perkins, et al., 1998

4

Property and functional loss

Property and functional loss

life safety

5

Woodframe Project Goal

• Advance the (seismic) engineering of woodframe buildings and improve the efficiency of their construction for targeted performance levels

• 23 CUREE reports are available from 22 testing and analysis tasks – everything from full building to individual nail tested

Element 1: Testing and Analysis

6

• CUREE Pub. W-04: Woodframe Project Case Studies, ed. by G.G. Schierle

• CUREE Pub. W-09: Northridge Earthquake Field Investigations: Statistical Analysis, G.G. Schierle

Element 2: Field Investigations

Element 3: Codes & Standards

Element 4: Loss Estimation• CUREE Report W-18: Improving Loss Estimation

for Woodframe Buildings, Porter et. al.

• CUREE Pub. W-30a & W-30b: Recommendations for Earthquake Resistance in the Design and Construction of Woodframe Buildings.

In all 30 Numbered Publications:www.curee.org

7

• All of the presented research results, recommendations and findings need to be qualified as being representative of available data and limited tested configurations.

Qualifications of Results, Recommendations and Findings

Building Configuration:

Open Front Behavior

8

Open Front Vulnerability 1971 San Fernando

Open Front Vulnerability 1989 Loma Prieta

9

Open Front Vulnerability 1994 Northridge

UC Berkeley Apartment Building

10

Without Finishes – 0.50g PGA

Canoga Park, Large Component

-1

-0.5

0

0.5

1

0 5 10 15 20 25Time (s)

Acc

eler

atio

n (g

) PGA = 0.42g

With Finishes – 0.50g PGA

Canoga Park, Large Component

-1

-0.5

0

0.5

1

0 5 10 15 20 25Time (s)

Acc

eler

atio

n (g

) PGA = 0.42g

11

Open Front ApartmentIndex Building

Open Front Torsional Response

12

Open Front Torsional Response

Without Finishes: Damage

13

Fractured Sheathing Nails

With Finishes: Damage

14

What Do We Do About Open Front inNew Construction?

• Open front configurations are inherently more vulnerable – avoid when possible

• Open front configurations are still permitted by code for new buildings

• Special attention is needed to make sure that walls perpendicular to the open front are design for combined direct and torsional forces

What Do We Do About Open Front inNew Construction?

• Increasing stiffness of walls perpendicular to open front should aid in reducing drift at open front face

• Consider providing extra strength and detailing

• NEHRP Proposal from last cycle available (was not adopted)

15

Moment Frame Retrofit

What Do We Do about Open Front inRetrofit of Existing?

• Life safety performance can be achieved using a SMRF along the open side designed using R = 4.5 with forces based on tributary area distribution

• Using an OMF appears acceptable, when R is set to be equal to 1 (check with building official)

• Check walls perpendicular to open front for required strengthening

16

What Do We Do about Open Front inRetrofit of Existing?

• Attention is needed to connection of frame to existing building:–Use connection with minimal slip

(steel to wood preferred over wood to wood

–Design for Ωo overstrength forces or larger

Building Configuration:

Soft & Weak Story Behavior

17

• Stiffness in one story is significantly less than other stories.

• Building deformation is concentrated in a single story during the earthquake.

Soft Story

Weak Story

Soft and Weak Combined• Story can be both soft and weak, making

it particularly vulnerable.

• Strength in one story is significantly less than other stories.

• High earthquake forces can cause story collapse.

What is Soft Story Behavior?

• Stiffness in one story is significantly less than other stories.

• Building deformation is concentrated in a single story during the earthquake.

18

What is Soft Story Behavior?

Soft Story Behavior

• Stiffness in one story is significantly less than other stories.

• Building deformation is concentrated in a single story during the earthquake.

19

Weak Story Behavior

• Strength in one story is significantly less than other stories.

• High earthquake forces can cause story collapse.

Soft / Weak Story Behavior

• Many buildings have stories that are both soft and weak, making them particularly vulnerable.

20

How Do We Know Soft Stories Occur?Two-Story UC San Diego House (W-06)

Ground Motion RecordsCanoga Park, Large Component

-1

-0.5

0

0.5

1

0 5 10 15 20 25Time (s)

Acc

eler

atio

n (g

) PGA = 0.42g

Rinaldi, Normal to Fault

-1

-0.5

0

0.5

1

0 5 10 15 20 25Time (s)

Acc

eler

atio

n (g

) PGA = 0.89g

Test Level 5

Test Level 4

21

Without Finishes – 0.89g PGA

Rinaldi, Normal to Fault

-1

-0.5

0

0.5

1

0 5 10 15 20 25Time (s)

Acc

eler

atio

n (g

) PGA = 0.89g

With Finishes – 0.89g PGA

Rinaldi, Normal to Fault

-1

-0.5

0

0.5

1

0 5 10 15 20 25Time (s)

Acc

eler

atio

n (g

) PGA = 0.89g

22

How do We Know Soft Stories Occur?

0

5

10

15

20

25

0.0 0.5 1.0 1.5 2.0 2.5Floor/Roof Peak Displacement (in)

Heig

ht

Wh

ere

Measu

red

(F

t)

Poor Quality

Typical Quality

SuperiorQuality

The Soft Story is Related to Finishes and Partitions

• More and larger exterior wall openings at 1st

story• Fewer and shorter

interior “partitions” at first story

• Designated structural walls not a good indication

23

Cripple Walls

• No interior finish at cripple wall story

• No interior “partitions” at cripple wall story

• Designated structural walls not a good indication

R

elat

ive

Dis

plac

emen

t (i

n) Phase 10 - With Wall Finish Materials Phase 9 - Without Wall Finish Materials

Time (s) Time (s)

1.04 in

-1.16 in

-6.00

-4.00

-2.00

0.00

2.00

4.00

6.00

0 5 10 15 20 25 30

2.71 in

-4.37 in

-0.29 in

-6.00

-4.00

-2.00

0.00

2.00

4.00

6.00

0 5 10 15 20 25 30 R

elat

ive

Dis

plac

emen

t (i

n) Phase 10 - With Wall Finish Materials Phase 9 - Without Wall Finish Materials

Time (s) Time (s)

1.04 in

-1.16 in

-6.00

-4.00

-2.00

0.00

2.00

4.00

6.00

0 5 10 15 20 25 30

2.71 in

-4.37 in

-0.29 in

-6.00

-4.00

-2.00

0.00

2.00

4.00

6.00

0 5 10 15 20 25 30

The Soft Story is Related to Finishes and Partitions

24

The Soft Stories are Related to Finishes and Partitions

0

5

10

15

20

25

0.0 0.5 1.0 1.5 2.0 2.5 3.0Floor/Roof Peak Displacement (in)

Hei

ght W

here

Mea

sure

d (F

t)

Phase 9 -Level 4

Phase 10 -Level 4

Phase 10 -Level 4R

Configuration Vulnerability Increased Due to Limited Finish Materials

•Influence of finish materials can explain concentrations of deformation demand and observed vulnerable building configurations.

25

• Soft stories are not necessarily bad• Soft stories are likely unavoidable • Need ductile detailing to accommodate

higher than normal drift in soft story– Plywood or OSB sheathing– Tie-downs, etc– Connecting posts top and bottom

• Recommend reasonable measures to stiffen and strengthen– Addition length of wall sheathing– Complete sheathing– Tie-downs and strapping

What Do We Do About Soft Stories?

• Weak stories are a concern in existing buildings where the strength is likely to be less than required by current code –these should be a priority for retrofit

• Weak stories are accommodated when complying to current code requirements –extent of resulting vulnerability not fully understood

• Recommend reasonable measures to strengthen weak story in new building

• Recommend design of shear and uplift anchorage for ΩO force level

What Do We Do About Weak Stories?

26

Building configuration:

Hillside Homes

Hillside Building Vulnerability

• Hillside sloping down from street front

• Cripple walls along sloping sides with tall (flexible) wall on downhill side

27

Hillside Building Vulnerability

• Post and beam (stilt system) on three sides, or only along downhill side in conjunction with stepped cripple walls along sloping sides

Hillside Building Collapse

28

Hillside Building Collapse

Hillside Base Diaphragm Response

29

Hillside Base Diaphragm Response

Diaphragm Anchorage Vulnerability

30

• Retrofit: City of Los Angeles Ordinance-Provides analysis methods & requirements for anchorage of base-level diaphragm to the uphill foundation-Limits story drift below base-level to 0.005h-Allows stepped cripple walls if variable rigidity of wall is considered, in-lieu of base anchorage, but does not specifically address additional loads to the stepped wall resulting from torsion

What Do We Do About Hillside Homes?

• New: Same approach – anchor framing directly to uphill foundation wall to avoid brittle failure

• NEHRP Proposal from last cycle available (was not adopted)

What Do We Do About Hillside Homes?

31

Base Diaphragm Primary Anchor

Force Distribution and Glued Floor Sheathing

32

• To minimize floor squeaks• Industry recommendations

Why Do We Glue Sheathing?

• Makes the floor more rigid and brings up issue of need for much more complex “rigid diaphragm” distribution of horizontal earthquake forces.

Why Do We Question Glued Sheathing?

33

What Does Testing Say About Effect of Gluing?

• Adhesives increase the shear stiffness of the diaphragm an average of 25%

• Blocking increases the shear stiffness of the diaphragm an average of 100%

• Adhesives further increase the effectiveness of blocking

• Adhesives do not fundamentally change the behavior of floor systems and should not be restricted based on added stiffness

What Does Testing Say About Effect of Gluing?

34

• Based on available data, it appears that better building performance will generally result when seismic forces can be resisted locally rather than requiring redistribution to other portions of the building. For this reason tributary area analysis is recommended for the great majority of buildings.

• Drift limits should be applied.

What Do Testing and Analysis Say About Need for Rigid

Diaphragm Force Distribution?

Interim Recommendation:

• Continue to glue floors is accordance with industry recommendations

• Use flexible diaphragm analysis unless building is significantly irregular (i.e. open front)

• Check for building department policy on force distribution – some enforcing Tri-Chapter recommendations.

What Do We Do?

35

7 Steps Towards Detailing for

Improved Shear Walls

1. Complete Exterior Sheathing (Spandrel Sheathing)

36

1. Complete Exterior Sheathing = Significant Change inDisplacement, Force

With Spandrel Sheathing

Without Spandrel Sheathing

Percent Change

Phase 6 6ATest Level 3 3Frequency (Cycles/Sec) 4.05 3.22Period (Sec) 0.25 0.31 26Peak 2nd Floor Displacement (In) 0.47 0.86 83Peak Roof Displacement (in) 0.89 1.95 119Peak Tie-Down Force (Kips) 1.6 4.0 150

2. Symmetrical Sheathing Edge Nailing Pattern

37

2. Symmetrical Sheathing Edge Nailing Pattern

• Non-symmetrical pattern shifts the Center of Rotation of each sheathing panel

• Damage was concentrated mainly at the edges having only a single row of nails

• Non-symmetrical patterns resulted in wall peak capacity, energy absorption and deformation capacity that was reduced when compared to symmetrical nailing

3. Steel Plate Washers for Foundation Sill Plate Anchorage

• Plate washers required by the 1997 UBC and NEHRP because of sill plate splitting observed in post-earthquake damage and from cyclic testing of shear walls at VT

• Plate washers were used in California Schools long before the 1997 UBC

• Plate washer size and thickness varies between codes (IBC, IRC, NEHRP) and sizing was not based on testing, nor was effectiveness ever quantified

38

Sill Plate Splitting in Anchorage Testing

Cross Grain Tension Mechanism

39

Combination of Splitting Mechanisms

• Uplift at wall ends from overturning creates weak axis bending

• Sheathing nails into the sill plate (near sheathing panel ends) create cross grain tension - Similar to roof ledger failure mode in Concrete Tilt-upsHeavily nailed walls will see the largest prying action effects, particularly from the nails nearest the ends of each panel

3. Steel Plate Washers for Foundation Sill Plate Anchorage

• Sill splitting reduces wall capacity and increases wall deflectionLarger size and thickness plate washers are needed at locations within 24 inches of the ends of sheathed walls

• Use UBC / IBC plate washer sizes as a minimum

• Will delay splitting in walls without tie-downs.

• Will likely eliminate splitting in walls with stiff tie-downs.

40

4. Increase shear wall nailing edge distance

• The larger the edge distance the tougher the wall will be

• Increases the displacement capacity and reduces the brittleness of the final failure

• 3/4 inch at top and bottom of wall and 5/8 for 3x framing members

5. Avoid over-driving of shear wall nailing

• Ensure that the nails are not over-driven in the sheathing (Driving depth is controlled more by the length of the driving pin in the nail gun than it is by the air pressure)

41

6. Increase member size at tie-downs and vertical sheathing

joints• Use 4x at tie-down end post

• Use 3x or 4x at vertical framing joints

7. End-stud stitch nailing

• If using double-stud as end post for shear walls, make sure that stud with sheathing edge nailing is adequately nailed to stud with tie-down attached

42

Nailing

Questions

43

Detailing for Steel Moment Frames

Moment Frame in Test Phase II

44

Moment Frame Component Test

Bolt Slip in Connection using Blocking

45

Bolt Damage in Blocking Connection

Conclusions and Recommendations

• Particular attention must be paid to limiting the slip between the moment frame beam and the wood structure –Alternate details

•

• Bolted connections at this interface should be designed for the maximum expected forces (e.g., special load combination) rather than normal code level forces

46

Horizontal Distribution of Earthquake Forces:

Rigid Versus Flexible Diaphragm

• There is no single characteristic stiffness for a diaphragm or shear wall; both have a wide range.

• Therefore there is no single correct solution for horizontal distribution of forces.

• Building performance is the ultimate goal.

Horizontal Distribution of Demand

47

• The concept of an exact solution needs to be abandoned, and adequate performance needs to be sought.

Horizontal Distribution of Demand

Horizontal Distribution Data

1. UC San Diego house demonstrated ability to redistribute forces, when going from a symmetrical to an asymmetrical configuration. CSIRO building similar.

48

Horizontal Distribution Data

2. Analyses of Large House Index Building with both flexible and rigid diaphragm designs, showed modest increase in drift and damage costs with rigid diaphragm design.

Horizontal Distribution Data

3. UC Berkeley apartment building illustrated that transferring forces across the building rather than resisting them locally can lead to significant deterioration of walls called on to resist the torsion.

49

4. The effect of finish materials greatly complicates the evaluation of wall stiffness. Stiffness varies greatly over loading history as well as with varying materials.

Finish Boundary Conditions

Influence of Finish Materials

Horizontal Distribution Data

Horizontal Distribution Data

5. Code deflection equations based on the bare wood structure tend to moderately to significantly overestimate maximum building deflections. If slender walls are designed to meet code drift limits, then the building overall drift should be somewhat less, resulting in adequate life-safety performance.

50

• Based on available data, it appears that better building performance will generally result when seismic forces can be resisted locally rather than requiring redistribution to other portions of the building. For this reason tributary area analysis is recommended for the great majority of buildings.

• Drift limits should be applied.

Interim Recommendation

51

• For buildings where the distribution of shear walls suggests a very significant torsional irregularity, use of a rigid diaphragm analysis or an envelope of rigid and flexible force distributions will be necessary.

• This is particularly true where a single wall line will carry the bulk of redistributed forces.

Interim Recommendation