| 摘要: | 國際民航組織（International Civil Aviation Organization, ICAO）為有效的評估不同航機對於現有鋪面之影響，以及了解現有鋪面之有效承載能力，於1983年公佈ACN/PCN方法。近年來，美國聯邦航空總署（Federal Aviation Administration, FAA）提出AC 150/5335-5B初稿做為PCN評估之依據，本研究主要針對各PCN評估方法之發展理論背景，以及各影響PCN決定之因子進行探討。 研究中首先針對各單位以往所提出之PCN評估方法進行回顧，並探討航機種類、輪軸荷重、路基土壤強度以及混合交通量等因素，對於PCN評估之影響。並在研究中亦評估利用CDF方法（cumulative damage factor method）進行等額交通量轉換之流程的詳細計算與分析。不論是舊有的PCN決定方法或是新的CDF方法，皆受到等額交通量轉換、不同應力模式採用以及路基土壤強度極大之影響。 航機分類指數之定義為：根據不同鋪面類型及路基強度，在不需指定鋪面厚度下，以此數值來表示一單獨飛機對鋪面的影響。航機分類指數為在不同路基土壤等級下之推導單輪重，在其決定過程中利用參考厚度之觀念，利用荷重來決定航機對鋪面之影響，因此省略鋪面之厚度。一般而言鋪面厚度對於鋪面應力之影響遠大於路基土壤強度。因此在研究中以此觀念做為出發點，建議一標準路基土壤等級（B級）做為後續ACN評估之依據。 機場鋪面分類指數之定義為：不指定任一特殊機型下，以此數值來表示鋪面承載荷重的能力；亦即不需任何起降限制條件下，機場鋪面所能提供之承載強度數值。在PCN評估過程中限制了允許工作應力為399 psi（2.75 MPa）。就混凝土疲勞以及航機重覆載重次數關係而言，當應力比（允許工作應力與混凝土抗彎強度之比值）很小時，其航機之起降次數可為無限次。而ACN/PCN之評估流程即以此為主要發展理念。因此在PCN評估時可不考量航機起降次數，僅需考量鋪面強度之影響。 本研究針對所發現之問題，以鋪面力學與鋪面疲勞效應做為出發點，建議一雙輪單軸重180,000 lbs之標準航機，配合Lee et. al所發展之中央荷重應力預估模式，建立各航機與標準航機之等額起飛次數轉換因子。並配合前述標準路基土壤（B級）建立一「改良式機場剛性鋪面PCN評估流程」，其中包含考量交通量以及不考量交通量之評估方法。 在現地鋪面結構強度代表值評估方面，本研究以統計方法為理論基礎，採用中央極限定理以及信賴區間（95%信心水準），並配合上述建立之等額起飛次數轉換因子，利用標準路基土壤強度之理念簡化PCN評估之代碼。在經由現地實際案例分析與驗證後，本研究所建議之方法相較於現有之PCN評估方法，確實可得到一穩定之PCN評估數值。 The International Civil Aviation Organization (ICAO) has adopted the Aircraft Classification Number / Pavement Classification Number (ACN/PCN) method as the standardized method for reporting airfield pavement bearing strength since 1980’s. Recently, the Federal Aviation Administration (FAA) has been circulating a draft Advisory Circular 150/5335-5B to provide specific guidance and revisions on how to arrive at a more reliable PCN. The primary objectives of this study are to investigate its fundamental principles, the reasoning of the newly-proposed revisions, and the effects on the PCN determination. The original development of ACN/PCN methodology and several rigid airfield pavement design procedures were first reviewed. Parameter studies on the effects of ACN/PCN determinations due to different aircraft types, gear loads, subgrade strengths, traffic mix, etc. were subsequently conducted. The newly revised approach by introducing a cumulative damage factor method for computing PCN based on equivalent traffic was also discussed. The results indicated that the PCN method is significantly influenced by the conversion of equivalent annual departures of the selected critical airplane, interior or edge stress determination, and a representative is-situ subgrade modulus for both the existing approach and the newly revised approach. ACN is originally defined as a number expressing the relative effect of an airplane at a given weight on a pavement structure for a specified standard subgrade. The concept of a single wheel load was employed to define the interaction of various gear loads and pavement without specifying pavement thickness as an ACN parameter. Since pavement thickness generally has much higher structural effect than subgrade strength, subgrade strength can be omitted along with pavement thickness when determining the relative effect. It was therefore proposed that a specific subgrade category (say “B”) be used for ACN determination. PCN is originally defined as a number expressing the load-carrying capacity of a pavement for “unrestricted operations” and a concrete working stress of 399 psi (2.75 MPa) was also assumed in this ACN/PCN approach. What were “unrestricted operations” really meant? According to the specified concrete fatigue relationship, unlimited number of load repetitions is achievable when the stress ratio (defined by the ratio of critical stress and flexural strength) is relatively small. In addition to no weight restriction (up to maximum takeoff weight), unrestricted operations are preferably considered to be able to sustain “unlimited number of load repetitions” as well. As such, a standard aircraft with a dual wheel gear load of 180,000 lbs was introduced. For consistency consideration, interior stress prediction models developed by Lee, et al. was used for critical stress estimations. An equivalent annual departure conversion factor was developed in this study using the concept of cumulative damage factors. A specific subgrade category (say “B”) was adopted. A modified PCN determination approach either with or without traffic mix consideration was subsequently developed. Based on the results of nondestructive testing, the concepts of random sampling, central limit theorem, and confidence intervals for hypothesis testing were adopted to determine various design inputs. It was also proposed that a single representative design input for the entire runway pavement be determined by the lower limit of 95% confidence level to derive a more consistent and repeatable PCN value. Several case studies were conducted to illustrate the potential problems of the existing PCN procedures and the benefits of the proposed revisions. |
