The Initial Assessment of Earthquake Prone Buildings:

A Wairarapa experience

What is an Earthquake Prone Building?

• A building is EQ prone if its ultimate capacity is exceeded in a moderate EQ at that site and likely to collapse involving life safety or damage to other property

• A moderate EQ is one third as strong as the EQ shaking used to design a new building at that site

• Generally does not apply to buildings used wholly for residential purposes

For details see Building Act 2004 Section 122 and Regulations

Seismicity of the Wairarapa

Extract from ‘Wairarapa Fault Trace Project’ report, URS Report to GRWC, 2006

Masterton Fault

Masterton District Policy

• MDC adopted an active approach

• Onus is on building owners to have their buildings assessed

• If a building is an EPB, the level to which it needs to be strengthened is 50%NBS

• Buildings which have had previous strengthening works have generally been excluded from the current review (Assessed in 1988 under Section 624 of the Local Government Act 1974)

• A timeframe of 10 years maximum is set to remove the EQ risk

Assessment Process

Assessment Process

• Step 1 – Research structure

• Step 2 – Visit the site

• Step 3 – Complete the relevant assessment

Step 1 -Research the Structure

•Review all information held by local authorities, ie district councils & take copies

•Compile a chronological history of the structure

•Note any pertinent features or structural unknowns to be reviewed on site

• Check that the building structural elements are as per the plans & note any changes

• Note the general condition of the building and any deterioration

• Photograph everything!

Step 2 - Visit the Site

Step 3 – Complete the relevant assessment

• Various methods to assess a structure depending on the type of structure

– IEP from the NZSEE

– Appendix 3 assessment from the NZSEE

– Specific Simple Calculations

NZSEE Initial Evaluation Procedure

• Provides a quick procedure for assessing a structure

• Based on the structure type and age

• Modified by several factors

NZSEE – Initial Evaluation Procedure (IEP)

Determine (%NBS)nom

Basedon

• Year constructed

• Soil Type

• Estimated period

Determine (%NBS)baseline

Based on (%NBS)nom x AxBxCxDxE factors

A – derived from Near fault factor N(T,D)

B – derived from Hazard Factor, Z

C – derived from return period

D – ductility scaling factor

E – Structural performance factor

Assess Performance Achievement Ratio –PAR

Based on

• Plan Irregularity

• Vertical Irregularity

• Short Columns

• Pounding Potential

• Site Characteristics

• Other Scaling Factors

Determine %NBSequals

(%NBS)baseline x PAR

NZSEE Appendix 3B – IEP for URM Buildings

• Provided to assess unreinforced masonry buildings built prior to 1935

• Both qualitative assessment of structure condition and quantitative assessment of structure characteristics are used

Simple Calculations

• May be best for some structures when compared with the NZSEE procedures

• Can include calculations from first principles

• For light timber framed structures can simply be a bracing plans with estimated bracing units for the materials present

Detailed Examples

• Example 1 - NZSEE Initial Evaluation Procedure Hunting & Fishing Building

• Example 2 – Simple Calculations

Seminis Seeds Storage Warehouse

Example 1 – Hunting & Fishing Building

• Originally constructed late 1800’s as two storey brick• Repair works were completed on the building after it was damaged in the

1934 earthquake, involved the installation of steel bracing in the roof and wall spaces and a central concrete pier in the front

• The building was entirely reconstructed after damage in the 1942 quakes. New concrete foundations and wall columns were constructed, with brick infill panels

Masterton 1897

Masterton 1906

1942 Earthquake Damage

1942 Earthquake Damage

• Review Masterton District Council records for building –some searching needed!

• In this example, the building is at 97 Queen St, however records were located in the folder for 99 – 103 Queen St

• Locate notes on building after 1942 EQ (if available) & search archives

• Compile chronological history of the structure

Example 1 - Document Search

Site Visit

•Street frontage

•Rear of building

•Side of building

•Reinforced concrete columns and beams constructed in 1943 visible

Site Visit

•Two cast iron columns providing gravity support to front

•Cast Iron columns were used between 1780’s – 1910 approx (NZSEE Assessment & Improvement of Structural Performance of Buildings in EQ – App 4A)

•Brick piers present elsewhere

•1934 drawings indicate a ‘New concrete pier secured to front wall’ in this location. No details on reinforcing or the foundation available

Site Visit

•View of side of front facade

•First 1943 reinforced concrete frame set back from facade

•Facade as per original construction, with the exception of the concrete pier addition in 1934

Site Visit

IEP Assessment – Step 4,5,6

Percentage of New Building Standard (%NBS)Step 4

Longitudinal Transverse

•(%NBS)b 21.4 21.4

•PAR 2.5 1.0

•PAR x (%NBS)b 53.5 21.4

Step 5 – Potentially Earthquake Prone?

%NBS<33 – YES

Step 5 – Potentially Earthquake Risk?

%NBS<67 – YES

Hunting & Fishing Building

• IEP assesses this building as being earthquake prone

• IEP results ‘forced’ by ‘F’ Factor

• Rationale for ‘F’ Factor relates to the facade

Hunting & Fishing Building•Longitudinal Direction – reasonably regular reinforced concrete frames with brick infill panels

•While it is not a ductile frame, there should be significant energy absorption and damping available in the form of cracking of the brick infill panels

•Transverse Direction – Concrete frames present at the rear and one other intermediate location, offset to the rear in the building

•Towards the front of the building –Reinf Conc U frames at approx 4.1m centres. These frames are not as rigid as the frames at the rear & intermediate location

•This produces an irregularity – leads to increase transverse deformations at the front of the structure

Note the first 1942 RC frame is set back from the facade – the facade structure remained unchanged

Hunting & Fishing Building•Front facade gravity loads are supported by a characteristically brittle system – bricks & cast iron!•Deformations are likely to affect the stability of the gravity support system of the facade

Recommended future investigation to determine1.Transverse capacity of the U frames2.Whether deformations of the frames at the Queen St frontage are likely to affect the stability of the facade

Note the difficulty with this – no drawings or information on entire facade structure. Foundations, connections etc?

Example 2 – Seminis Seeds Warehouse

• No information available for the structure• Built around 1920’s• 61m x 15m Timber building with 19 timber frames at

approx 3.1m centres• Transverse frames are timber trusses and columns of

dual studs and bolted knee braces at the truss-column joint

• Walls & roof close sarked timber with CGI roofing• Minor addition for internal plant, and a new internal

office added

Site Visit

•External view

Internal View

-Note missing knee braces

Site Visit

Step 2 – Assessment of Structure• NZSEE IEP not the best tool for the assessment of this type of structure

• Specific simple calculations required

• Using AS/NZS1170, EQ & Wind loads on the structure were derived;

Wind Earthquake

AS/NZS 1170 Load

Equivalent Gust speed

AS/NZS 1170 Load

33% NBS Equivalent Wind Gust speed of 33%

NBS

Transverse (load per frame)

13kN38m/s

(148km/hr)12.6kN 4.2kN

22m/s (79km/hr)

Longitudinal (load per wall) 45kN

38m/s (148 km/hr)

120kN 40kN35m/s

(126 km/hr)

• In both directions 33% of the current standards EQ loading is less than the wind loading

Transverse Analysis•NIWA data for the Masterton area documents a mean annual wind gust speed event of greater than 25.8m/s on two days per year

•From this we can assume that the buildings transverse capacity is greater than 33% NBS

Longitudinal Analysis•In this direction, forces resisted by two external walls

•Two sets of data used

•Superseded Light Timber Framed Code 3604:1990 Appendix K Table K1 – gives a bracing unit for close timber sarking

•First principles check calculation on the strength of the sarking. Calculated the likely shear capacity of the sarking, determined by nail connections

•Provided a value of 2.1kN/m

•With 61m length of wall, resistance provided is 128kN per wall, which is greater than 33% NBS

Knee braces reinstated

Notes on the NZSEE IEP

IEP – the maths

Pre 1965 in Masterton locale with period <0.4sec

(%NBS)b=4%

1965-1976 in Masterton locale with period <0.4sec

(%NBS)b=7.5%

IEP Factors – Masterton Urban

• Step 2 factors A, B & C relate to location

For standard buildings in Masterton,

AxBxC always = 2.38

• Step 2 factors D & E rely on the structure ductility,

– µ=2, then DxE=2.245

– µ=1, then DxE=1.0

• If µ=2, AxBxCxD=5.343

• If µ=1, AxBxCxD=2.38

The ‘F’ FactorDuctility (%NBS)nom

Product of factors for baseline

(%NBS)b PAR (excl.

Factor ‘F’)

Req’d ‘F’ to >33%

Pre

1

93

5 µ=13.2%

X 2.38 = 7.62% 1 4.4

µ=2 x 5.38 = 17.22% 1 2.0

19

65

-1

93

5 µ=1

4%x 2.38 = 9.52% 1 3.5

µ=2 x 5.38 = 21.4% 1 1.6

19

65

-1

97

6 µ=17.5%

X 2.38 = 17.85% 1 1.9

µ=2 x 5.38 = 40.35% 1 0.9

This Value is set, ie no judgement involved

This value generally set, period and ductility are the only variables

The PAR for a building with no CSW’s is set entirely by the ‘F’ Factor

Max ‘F’ for <2 storeys=2.5, no min

‘F’ factor will determine if buildings ‘pass’ or ‘fail’

‘F’ Factor

• Basically the assessment comes down to the judgement of the engineer

• While pounding, plan and vertical irregularity are a factor, buildings in our locale generally ‘pass’ or ‘fail’ determined by ‘F’ factor

• Nominal guidance on the appropriate application of ‘F’ Factors in the NZSEE guide

‘Well designed and constructed regular building of its type and vintage on the site in question..would have a PAR = 1’

NZSEE Recommendations

Quick ExamplesMalmos’ Garage – Present Day

Malmos’ Garage – 1920’s• Originally 2

storey brick

• Chapel St front badly damaged in 1942 EQ

Malmos’ Garage 1960’s

• ‘Strengthened’ in 1953

Sports Arena

Conclusions

1. The necessity of the Building Act requirement to evaluate the seismic capacity of all buildings constructed prior to the introduction of modern codes and not only URM’s is quite clear and supports the long held view of NZSEE

2. The initial evaluation procedure outlined in the Recommendations of the NZSEE Study Group on Earthquake Risk buildings is a practical workable basis on which to initially assess the seismic performance of ERB’s and should produce relatively uniform seismic grading outcomes. It is in effect a ‘coarse sieve’

3. The IEP outcome is very sensitive to the Factor F used in determining the PAR. Factor F is entirely based on the judgement of the assessor and has a significant affect on the % NBS achieved and the seismic grading

Conclusions...cont’d

4. In Masterton, a good (µ=2) pre-1935 buildings requires an F value of 2.0 to achieve a %NBS greater than 33

5. Single storey timber framed structures which do not fit comfortably into the IEP procedure can be evaluated using simple calculation procedures, such as bracing units, to produce a %NBS value

6. Life safety and the likelihood of damage to any other property are critical requirements

7. The time taken to carry out an IEP assessment is generally around 10-15 hours8. Having a very experienced local Engineer to consult with is a valuable resource!

Historical information on construction methods and materials over time is very beneficial

Acknowledgements

Special thanks to George Butcher

Thankyou