Bridge Condition Indicator Guide 8 pages

technical memorandum
bridges and other highway structures series
Bridge condition indicator guide
TM-4008
March 2014
1.0 Introduction
1.1
The need for bridge indicators
In order to maintain and manage a stock of bridges and other highway structures, it is
essential to have a “condition indicator” that can be used to determine whether the overall
condition of the highway structures is deteriorating or not, and use this as a means for
monitoring whether adequate funding is being provided for bridges and other highway
structures maintenance work.
1.2
Derivation
The basis of the derivation is taken from the UK County Surveyors Society (CSS) guides
Bridge condition indicators published in 2002( 1) and 2004( 2). These guides are no longer
available in print, so a summary of the salient points is reproduced below.
2.0 Application
2.1
Inspection
The bridge inspector carries out the inspection in accordance with NZTA S6 Bridges and
other highway structures inspection policy( 3), utilising the appropriate inspection proforma
from the policy. The bridge inspector records the defects of the individual bridge elements
on the inspection forms.
2.2
Bridge score
The inspection results of severity (1-5) and defect extent (A-E) for each element of the
structure are recorded by the bridge inspector on the condition rating spreadsheet from
the individual inspection records.
2.3
Network area and national bridge index score
A bridge stock condition index (BSCI) for both the average and critical conditions can be
provided for a network area or the complete state highway network.
3.0 Summary of the CSS Bridge Group Bridge condition
indicators(1)(2)
3.1
Acronyms
Below is a list of the acronyms used in the derivation of the bridge condition indicators:
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highway structures series
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3.2
BCI
Bridge condition index: numerical value of a bridge condition evaluated
using the BCS on a scale of 100 (best condition) to 0 (worst condition)
BCS
Bridge condition score: numerical value of a bridge condition on a scale of 1
(best condition) to 5 (worst condition)
BCSAv
Average bridge condition score for a bridge evaluated taking into account
the condition of all structural elements in a bridge
BCSCrit
Critical bridge condition score for a bridge evaluated taking into account the
elements deemed to be of very high importance in a bridge
BCIAv
Average bridge condition index for a bridge evaluated taking into account
the condition of all structural elements in a bridge
BCICrit
Critical bridge condition index for a bridge evaluated taking into account the
elements deemed to be of very high importance in a bridge
BSCI
Bridge stock condition index: the numerical value of a bridge stock
condition evaluated as an average of the BCI values weighted by the deck
area (m2) of each bridge
BSCIAv
Average bridge stock condition index evaluated using the BCIAv values for all
bridges in the stock
BSCICrit
Critical bridge stock condition index evaluated using the BCICrit values for all
bridges in the stock
ECI
Element condition index: the weighted element condition taking account of
the ECS and ECF
ECS
Element condition score: numerical value of the condition of an element
evaluated using inspection data (severity and extent) on a scale of 1 (best
condition) to 5 (worst condition)
EI
Element importance: to take account of the importance of an element to the
overall bridge in terms of load carrying capacity, durability and public safety.
Designated as low, medium, high or very high. Element importance
classification is used to identify two factors, namely the ECI and the ECF
ECF
Element condition factor: used to weight the ECS to obtain the ECI. This
enables direct comparison of element conditions in terms of their
contribution to the overall bridge condition
EIF
Element importance factor – used to weight individual ECI scores when
evaluating the BCSAv
Deck area
(overall width) x (distance from centreline to centreline of end supports) or
(distance between face of end supports + 0.6m)
Overview of the procedure
The overall procedure for producing the condition indicators is given below, including a
flow chart diagram in figure 1.
(i)
Each element within a bridge is selected in turn and its condition data is used to
produce an element condition score (ECS) for the element.
(ii)
Next, the element importance (EI) is identified, this accounts for the importance of
the element to the overall condition and functionality of the bridge. Then the element
condition factor (ECF) is evaluated by taking into consideration the element
importance and the ECS.
(iii)
The ECS and ECF values are combined to produce the element condition index (ECI)
which represents the condition of the element on a scale of 1 (best) to 5 (worst).
Steps (i) to (iii) are repeated for all elements in a bridge.
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highway structures series
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3.3
(iv)
Next, two different bridge condition scores are evaluated: BCSAv is an average of ECI
values of all the elements in a bridge (weighted by the element importance factor,
EIF), and BCSCrit is the maximum of ECI values of those elements which are considered
“critical” to the integrity of the bridge. BCS values therefore have the same 1 to 5
scale as ECI.
(v)
The BCS values are then converted to the corresponding bridge condition indices BCIAv
and BCICrit on a scale of 100 (Best) and 0 (Worst) condition. Steps (i) to (v) are repeated
for all bridges in the stock.
(vi)
Finally, the BCI values for all bridges in the stock are weighted by their respective
deck areas and the average values for the stock are evaluated. Thus the bridge stock
condition index BSCIAv is a weighted average of BCIAv values, while the BSCICrit is a
weighted average of BCICrit values for all bridges in the stock. BSCI values have the
same 100 (Best) to 0 (Worst) scale as BCI.
Flow chart process for deriving the bridge condition index (BCI)
Figure 1 is taken from Bridge condition indicators volume 3 Guidance note on evaluation of
bridge condition indicators(1).
Figure 1: Flow chart for the derivation of bridge condition indicators
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highway structures series
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NZ Transport Agency 2014
3.4
Inspection definitions
The definitions for reporting the condition defects to bridge elements are as follows:
3.5
Extent
The area, length or number (as appropriate) of the bridge
element affected by the defect/damage
Severity
The degree to which the defect/damage affects the
function of the element or other elements on the bridge
Extent codes
The extent codes are used to record the percentage amount of the element which is
affected.
Table 1: Extent codes
Code
3.6
Description
A
No significant defect
B
Slight, not more than 5% of surface area/length/member
C
Moderate, 5% - 20% of surface area/length/number
D
Wide, 20% - 50% of surface area/length/number
E
Extensive, more than 50% of surface area/length/number
Severity descriptions
The severity description codes are used to record how severe the defect of the element is.
Table 2: Severity descriptions
Code
3.7
Description
1
As new condition or defect has no significant effect on the
element (visually or functionally)
2
Early signs of deterioration, minor defect/damage, no
reduction in functionality of element
3
Moderate defect/damage, some loss of functionality could
be expected
4
Severe defect/damage, significant loss of functionality
and/or element is close to failure/collapse
5
The element is non-functional/failed
Permissible combinations
These are the combinations that are permitted during the inspection process.
Table 3: Permissible combinations of severity and extent
Extent
Code
Severity
1
2
3
4
5
A
A1
B
B1
B2
B3
B4
B5
C
C1
C2
C3
C4
C5
D
D1
D2
D3
D4
D5
E
E1
E2
E3
E4
E5
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highway structures series
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NZ Transport Agency 2014
3.8
Element condition score (ECS)
The element condition score is attributed to the relevant severity-extent rating
combination.
Table 4: Element condition score (ECS)
Extent
code
Severity
1
2
3
4
A
1.0
B
1.0
2.0
3.0
4.0
C
1.1
2.1
3.1
4.1
D
1.3
2.3
3.3
4.3
E
1.7
2.7
3.7
4.7
5
5
The scoring values reflect that the extent of the damage is less critical than the severity of
the damage.
3.9
Element importance factor (EIF)
The element importance factor attributes a figure for each element depending on the
structural importance of the individual element.
Table 5: Element importance factor (EIF)
Element importance
EIF
Very high
2.0
High
1.5
Medium
1.2
Low
1.0
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highway structures series
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NZ Transport Agency 2014
The following is taken from the bridge inspection proforma record sheet from NZTA S6(3)
with each element importance description of very high, high, medium and low along with
the appropriate factor.
Table 6: Element importance classification for different bridge elements
Set
Item no.
Other
Retaining
elements
Waterway
elements
Safety
elements
Durability
elements
Load-bearing
substructure
Superstructure
elements
1
TM-4008 (03/2014)
2
3
Element description
Primary load carrying element
Secondary
element(s)
Transverse beams
Other (incl. deck)
4
Half joints
5
Seismic linkages/holding down bolts
6
Parapet beam or cantilever
7
Cross bracing
8
Foundations
9
Abutments
10
Head wall
11
Pier / column
12
Cross-head / capping beam
13
Bearings
14
Bearing plinth / shelf
15
Superstructure drainage
16
Substructure drainage
17
Movement / expansion joints
18
Painting: Superstructure elements
19
Painting: substructure elements
20
Painting: barriers/guardrails
21
Access / walkways / gantries
22
Guardrail / handrail / safety fences
23
Carriageway surfacing
24
Footway / verge / footbridge surfacing
25
Invert / river bed
26
Aprons
27
River bed upstream
28
River bed downstream
29
Scour
30
River banks
31
Revetment / batter slope paving
32
Wing walls
33
Retaining walls
34
Embankments
35
36
37
38
39
40
Approach rails / barriers / walls
Approach adequacy
Signs
Lighting
Services
Appearance
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highway structures series
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Element importance
EIF
Very high
Very high
Very high
Very high
Very high
Very high
High
High
High
High
Very high
Very high
High
Medium
Medium
Medium
High
Medium
Medium
Medium
Medium
High
Medium
Low
Medium
Medium
Medium
Medium
Medium
Medium
Low
High
Medium
Low
2.0
2.0
2.0
2.0
2.0
2.0
1.5
1.5
1.5
1.5
2.0
2.0
1.5
1.2
1.2
1.2
1.5
1.2
1.2
1.2
1.2
1.5
1.2
1.0
1.2
1.2
1.2
1.2
1.2
1.2
1.0
1.5
1.2
1.0
Elements not used to
evaluate condition
indicators, thus
importance not
required
NZ Transport Agency 2014
3.10 Element condition factor (ECF)
The element condition factor uses the element condition score to calculate a reduction
factor for elements of lesser structural importance.
Table 7: Expressions for element condition factor (ECF)
Element importance
Element condition factor (ECF)
Very high
𝐸𝐶𝐹 = 0
0.3
�
4
0.6
𝐸𝐶𝐹 = 0.6 − �(𝐸𝐶𝑆 − 1) ×
�
4
1.2
𝐸𝐶𝐹 = 1.2 − �(𝐸𝐶𝑆 − 1) ×
�
4
𝐸𝐶𝐹 = 0.3 − �(𝐸𝐶𝑆 − 1) ×
High
Medium
Low
ECS = range of 1 to 5 (1 = good, 5 = poor)
3.11 Element condition index (ECI)
The element condition index (ECI) indicates the contribution the condition of an element
makes to the condition of the bridge as a whole. The ECI is determined by adjusting the
element condition score (ECS) to account for the element condition factor (ECF) as shown
follows.
𝐸𝐶𝐼 = 𝐸𝐶𝑆 − 𝐸𝐶𝐹
but is always ≥1
3.12 Bridge condition score (BCS)
The average bridge condition score is calculated by using the element condition index and
the element importance factor.
The critical bridge condition score is taken as the highest value of the “very high” element
condition index.
𝐵𝐶𝑆𝐴𝑣 =
𝐵𝐶𝑆𝐶𝑟𝑖𝑡
∑(𝐸𝐶𝐼𝑖 × 𝐸𝐼𝐹𝑖 )
∑ 𝐸𝐼𝐹𝑖
ECI for primary deck elements
⎫
ECI for secondary deck elements
⎪
⎪
ECI for half joints
= max ECI for seismic linkage or holding down bolts
⎨
⎬
ECI for parapet beam or cantilever
⎪
⎪
ECI for pier⁄column
⎪
⎪
ECI for crosshead or capping beam
⎩
⎭
⎧
⎪
⎪
3.13 Bridge condition index (BCI)
The bridge condition index (BCI) is calculated by applying the following formulae:
𝐵𝐶𝐼𝐴𝑣 = 100 − 2[(𝐵𝐶𝑆𝐴𝑣 )2 + (6.5 × 𝐵𝐶𝑆𝐴𝑣 ) − 7.5]
𝐵𝐶𝐼𝐶𝑟𝑖𝑡 = 100 − 2[(𝐵𝐶𝑆𝐶𝑟𝑖𝑡 )2 + (6.5 × 𝐵𝐶𝑆𝐶𝑟𝑖𝑡 ) − 7.5]
Results: best = 100, worst = 0
TM-4008 (03/2014)
Technical memorandum: bridges and other
highway structures series
Page 7 of 8
NZ Transport Agency 2014
3.14 Bridge stock condition index (BSCI)
The bridge stock condition index is calculated as an average of the BCI values weighted by
the square metre of deck area of each bridge is calculated by applying the following
formulae for both the average and critical values.
𝐵𝑆𝐶𝐼𝐴𝑣 =
∑(𝐵𝐶𝐼𝐴𝑣 × Deck area)
∑ Deck area
𝐵𝑆𝐶𝐼𝐶𝑟𝑖𝑡 =
∑(𝐵𝐶𝐼𝐶𝑟𝑖𝑡 × Deck area)
∑ Deck area
4.0 Excel calculation file
The Transport Agency will provide the bridge management consultants with the current
excel file to assist with the calculation of the relevant values.
5.0 References
(1)
County Surveyors Society (2002) Bridge condition indicators. United Kingdom.
Volume 1 Commission report
Volume 2 Guidance note on bridge inspection reporting
Volume 3 Guidance note on evaluation of bridge condition indicators.
(2)
County Surveyors Society (2004) Addendum to CSS guidance note on bridge condition
indicators. United Kingdom.
Volume 2 Bridge inspection reporting
Volume 3 Evaluation of bridge condition indicators.
(3)
NZ Transport Agency (2014) NZTA S6 Bridges and other highway structures
inspection policy. Wellington.
TM-4008 (03/2014)
Technical memorandum: bridges and other
highway structures series
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NZ Transport Agency 2014

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