technical memorandum

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1 technical memorandum bridges and other Bridge condition indicator guide TM-4008 March 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 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: Page 1 of 8

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) BCS Av BCS Crit BCI Av BCI Crit BSCI BSCI Av BSCI Crit ECI ECS EI ECF EIF Deck area Average bridge condition score for a bridge evaluated taking into account the condition of all structural in a bridge Critical bridge condition score for a bridge evaluated taking into account the deemed to be of very high importance in a bridge Average bridge condition index for a bridge evaluated taking into account the condition of all structural in a bridge Critical bridge condition index for a bridge evaluated taking into account the deemed to be of very high importance in a bridge 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 (m 2 ) of each bridge Average bridge stock condition index evaluated using the BCI Av values for all bridges in the stock Critical bridge stock condition index evaluated using the BCI Crit values for all bridges in the stock Element condition index: the weighted element condition taking account of the ECS and ECF 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) 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 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 Element importance factor used to weight individual ECI scores when evaluating the BCS Av (overall width) x (distance from centreline to centreline of end supports) or (distance between face of end supports + 0.6m) 3.2 Overview of the procedure The overall procedure for producing the condition indicators is given below, including a flow chart diagram in figure 1. (i) (ii) (iii) 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. 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. 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 in a bridge. Page 2 of 8

3 (iv) (v) (vi) Next, two different bridge condition scores are evaluated: BCS Av is an average of ECI values of all the in a bridge (weighted by the element importance factor, EIF), and BCS Crit is the maximum of ECI values of those which are considered critical to the integrity of the bridge. BCS values therefore have the same 1 to 5 scale as ECI. The BCS values are then converted to the corresponding bridge condition indices BCI Av and BCI Crit on a scale of 100 (Best) and 0 (Worst) condition. Steps (i) to (v) are repeated for all bridges in the stock. 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 BSCI Av is a weighted average of BCI Av values, while the BSCI Crit is a weighted average of BCI Crit values for all bridges in the stock. BSCI values have the same 100 (Best) to 0 (Worst) scale as BCI. 3.3 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 Page 3 of 8

4 3.4 Inspection definitions The definitions for reporting the condition defects to bridge are as follows: Extent Severity The area, length or number (as appropriate) of the bridge element affected by the defect/damage The degree to which the defect/damage affects the function of the element or other on the bridge 3.5 Extent codes The extent codes are used to record the percentage amount of the element which is affected. Table 1: Extent codes Code A B C D E Description No significant defect Slight, not more than 5% of surface area/length/member Moderate, 5% - 20% of surface area/length/number Wide, 20% - 50% of surface area/length/number Extensive, more than 50% of surface area/length/number 3.6 Severity descriptions The severity description codes are used to record how severe the defect of the element is. Table 2: Severity descriptions Code Description As new condition or defect has no significant effect on the 1 element (visually or functionally) Early signs of deterioration, minor defect/damage, no 2 reduction in functionality of element Moderate defect/damage, some loss of functionality could 3 be expected Severe defect/damage, significant loss of functionality 4 and/or element is close to failure/collapse 5 The element is non-functional/failed 3.7 Permissible combinations These are the combinations that are permitted during the inspection process. Table 3: Permissible combinations of severity and extent Extent Severity Code 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 Page 4 of 8

5 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 A 1.0 Severity B C D E 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 Page 5 of 8

6 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 Set Item no. Element description Element importance EIF 1 Primary load carrying element Very high Secondary Transverse beams Very high element(s) Other (incl. deck) Very high Half joints Very high Seismic linkages/holding down bolts Very high Parapet beam or cantilever Very high Cross bracing High 1.5 Superstructure Load-bearing substructure Durability Safety Waterway Retaining Other 8 Foundations High Abutments High Head wall High Pier / column Very high Cross-head / capping beam Very high Bearings High Bearing plinth / shelf Medium Superstructure drainage Medium Substructure drainage Medium Movement / expansion joints High Painting: Superstructure Medium Painting: substructure Medium Painting: barriers/guardrails Medium Access / walkways / gantries Medium Guardrail / handrail / safety fences High Carriageway surfacing Medium Footway / verge / footbridge surfacing Low Invert / river bed Medium Aprons Medium River bed upstream Medium River bed downstream Medium Scour Medium River banks Medium Revetment / batter slope paving Low Wing walls High Retaining walls Medium Embankments Low Approach rails / barriers / walls 36 Approach adequacy 37 Signs 38 Lighting 39 Services 40 Appearance Elements not used to evaluate condition indicators, thus importance not required Page 6 of 8

7 3.10 Element condition factor (ECF) The element condition factor uses the element condition score to calculate a reduction factor for of lesser structural importance. Table 7: Expressions for element condition factor (ECF) Element importance Element condition factor (ECF) Very high ECF = 0 High ECF = 0.3 (ECS 1) Medium ECF = 0.6 (ECS 1) Low ECF = 1.2 (ECS 1) 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 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. BCS Av = (ECI i EIF i ) EIF i ECI for primary deck ECI for secondary deck ECI for half joints BCS Crit = 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: BCI Av = 100 2[(BCS Av ) 2 + (6.5 BCS Av ) 7.5] BCI Crit = 100 2[(BCS Crit ) 2 + (6.5 BCS Crit ) 7.5] Results: best = 100, worst = 0 Page 7 of 8

8 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. BSCI Av = (BCI Av Deck area) Deck area BSCI Crit = (BCI Crit 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. Page 8 of 8