The first phase of the study involved the execution of the following tasks for signal poles:

Transcription

1 Signal Pole, Sign Structure, and High Mast Lighting Structure Design The first phase of the study involved the execution of the following tasks for signal poles: Task 1 Conduct literature review and a survey from federal and other state agencies The focus of this phase was to locate, collect, and list all the available current state of thepractice methods for (1) FHWA s regulations, (2) other states practices, and (3) research and testing findings. A summary of this task is located in Chapter 2, Literature Review. Preliminary contacts were made to members of the AASHTO Subcommittee on Bridges and Structures, T 12 Structural Supports and TRB Committee on General Structures (AFF10). More details are discussed in Chapter 3, National Survey. Fabricators and design engineers were also contacted. Task 2 Develop scenario and work plans of the cantilevered mast arm signal structures In this task, a comparison was made between the yet adopted AASHTO LRFD Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals (LTS LRFD, 2015) and the existing MD SHA Book of Standards for Highway, Incidental Structures and Traffic Control Applications to make sure that the current AASHTO design criteria were met. A fatigue importance factor, IF, accounts for the risk of hazard to traffic and damage to property and was applied to the limit state wind load effects. Based on the latest AASHTO Specifications 6 th edition (LTS 6, 2013), fatigue importance factors for traffic signal and sign support structures were exposed to three wind load effects and are presented below in Table 1.1. The bolded importance factors, which are for cantilevered mast arm signal structures, were used for this study. Table 1.1 AASHTO Fatigue Importance Factors Fatigue Importance Category Cantilevered Non Cantilevered I II III I II III Sign Traffic Signal Sign 0.70 Traffic Signal 0.65 Sign 0.40 Traffic Signal 0.30 Sign Traffic Signal x x Sign Traffic Signal x x Sign Traffic Signal x x Galloping Natural Wind Gusts Truck Induced Gusts

2 Structures classified as Category I present a high hazard in the event of failure and should be designed to resist rarely occurring wind loading and vibration phenomena. It is recommended that all structures to be classified as Category I if there are no effective mitigation devices on the structures on roadways with a speed limit exceeding 60 km/h (35 mph), and a roadway which has average daily traffic (ADT) exceeding 10,000 or average daily truck traffic (ADTT) exceeding Structures should be classified as Category III if they are located on roads with speed limits 60km/h (35 mph) or less. Structures that are located such that a failure do not affect traffic may be classified as Category III. All structures not explicitly meeting the Category I or Category III criteria should be classified as Category II. Maintenance and inspection programs should be considered integral to the selection of the fatigue importance category. There are many factors that affect the selection of the fatigue category and engineering judgment is required. Reviews were made on the AASHTO LRFD Specifications (LTS LRFD, 2015) with previous 4 th to 6 th editions and MD SHA Standard for the best and most cost effective design. More details are discussed in Chapter 4, Design Criteria Based on AASHTO Specifications. Also, a discussion on the basis for using Category II in design is covered in Chapter 5, Vibration Mitigation Devices for Signal Poles. Task 3 Collect, analyze, evaluate and assess the current MD structure designs Representative samples of cantilevered mast arm signal structures were collected from the SHA inventory based on their categories, single or multiple arms, and level or curved arms. Based on the MD SHA Standards for Highway and Incidental Structures (Standard No MD800.01), leveled and curved cantilevers with four different arm lengths (50, 60, 70 and 75 ) were studied. Analyses were made on those MD structures based on the AASHTO load criteria and the results are tabulated for comparison in Chapter 6, Signal Pole Design. Task 4 Provide a cost analysis for recommending an economical and fatigue resistant design As previously mentioned, eight cantilevered master arm signal structures (four level and four curved cantilever with the arm lengths of 50, 60, 70, and 75 ) with different pole mounting and arm attachment details were studied. Fatigue details are shown in Figure 1.1, where the pole mounting, arm attachment, and access hole (all circled) were the concerns for fatigue details and relevant costs. To get a more accurate cost comparison, fabricators were consulted for cost analysis and an economical and fatigue resistant design was recommended. More details are covered in Chapter 6, Signal Pole Design Based on Maryland Assumptions and Chapter 7, Investigation of Maryland Signal Pole Foundation.

3 Figure 1.1 Cantilevered Mast Arm Signal Structure and Its Fatigue Detail Locations Task 5 Summary and Report A summary of all the four tasks listed above is included in Chapter 8, Summary and Conclusion. The report also summarizes the recommendations associated with an economical and fatigue resistant design and the study of fatigue resistant design criteria for MD SHA cantilevered mast arm signal structures. The enhanced SABRE program with fatigue loading analysis and an excelbased tool for the fatigue detail evaluation of the structures are also addressed. Training is planned and included.

4 Phase II of the study associated with the calibration of the old and new designs for sign and high mast lighting structures: Task 1: Complete design parameters of MD specified sign and light structures In the previous project, extensive studies have been conducted by this research team on determining the complete design parameters for MD specified signal structures. Similar task is conducted to identify the Maryland specified Fatigue Importance Category Factor (I, II or III) for sign and high mast light structure designs. The determination of the design parameters is based on (1) literature review based on the previous and current studies conducted by this research team, (2) previous survey conducted by Maryland for signal structures and Arizona for sign structures, (3) members of the AASHTO Subcommittee on Bridges and Structures, T 12 Structural Supports and TRB Committee on General Structures (AFF10), (4) Maryland requirement based on the determination of the SHA/ OOTS and this research team. Task 2: Determination of wind pressure based on the rationale of wind loads and their mean recurrence interval (MRI) defined in the AASHTO LRFD LTS Specs. Wind loads based on 2013 ASD (STD LTS) and 2015 LRFD are shown below where the highlighted ones are expressed differently between the two: a) 2013 wind pressure Pz= K z *G*V 2 *I r *C d b) 2015 LRFD wind pressure Pz= K z *K d *G*V 2 *C d Both formula have variable V where V, the wind speed, is now grouped as four MRI categories, 10yr, 300yr, 700yr and 1700yr. AASHTO LRFD LTS Spec. Table determines which MRI value should be applied (by ADT and risk). This change is likely to make up for the removal of I r. The current 100mph for all cases is no longer considered reasonable. On the other hand, the current (2013) V map is equivalent to 2015 LRFD 300yr wind map. AASHTO LRFD LTS Table Mean Recurrence Interval

5 As far as the roadside structure design is concerned, Maryland can be divided into three regions, Eastern Shore, Appalachia Mountain and the Baltimore Washington corridor. Each region is following its own wind map contour. The goal of this task is to determine what kind of wind speed or speeds should be used for structural support designs of Maryland highway signs, luminaires, and traffic signals. The remaining question is whether we should keep the legacy of wind speed and wind speed equation for the existing structural supports for the future evaluation purpose. Impact study is made on changing the wind speed(s) in designs. Task 3: Calibration of designs by adopting fatigue design defined in the AASHTO LRFD LTS Specs. and developing fatigue resisting connections Fatigue design was first covered in the AASHTO STD LTS 5 as load combination four (LC4). Fatigue resistance was then modified to the current form in both the AASHTO STD LTS 5 and LRFD LTS Specs. Section 11 in STD LTS 6 is conceptually identical to the LRFD LTS specifications. Maryland is on schedule to adopt the fatigue design for all structural supports of highway signs, luminaires, and traffic Signals. The main item is the adoption of fatigue design in all phases. Fatigue design is associated with fatigue details as shown in the AASHTO LRFD LTS Table Details of Maryland signal poles which need to be modified in order to increase the fatigue resistance have been identified and recommended by this University of Maryland Research Team. The recommended modifications of current design on the signal poles include: a) Groove welds for arm connections b) Groove welds for pole connections c) Adopting AASHTO build up box type for arm connections d) 6 bolt patterns for both arm and pole connections

6 Similar recommendations and their impacts of modifying details on sign structural supports are also studied in this proposed task. One example is the study of tube to tube connections between main chords and bracings for sign structures, i.e. choosing between pipe to pipe vs gusset plate weld connections for box truss sign structures. Task 4: Foundation design by the AASHTO LRFD LTS Specs. Fatigue design would not affect the support foundation, but wind load will. In the AASHTO LRFD LTS Extreme I, Service I and II Limit States have wind Load (W) involved and should be considered in foundation designs. In Maryland Book of Standard for Highways, Incidental Structures and Traffic Control Applications, three types of foundations are covered for their own respective types of structures: a) Shaft foundation for signal poles b) Shaft foundation with wing walls for cantilever sign structures c) Mat found with pedestal for overhead Different winds and load combinations covered by the AASHTO STD LTS and LRFD LTS are greatly affect the foundation designs. Preliminary study has been made by this University of Maryland Research Team on shaft foundation designs on cohesive and cohesionless soils by the AASHTO LRFD LTS. Slight modifications may be needed in order to cover the LRFD design on a Maryland standard 50 to 75 arm sign structures with setup of one road sign, two left turn signs and five signal heads. All the standard foundation designs covered in MD801.1 for signal structures, MD807 and MD for cantilever and overhead sign structures, respectively and may need modification. Representative samples of these three types are studied in this task.

7 Task 5: Comparison between the AASHTO STD LTS and LRFD LTS Specifications Subtask 5.1 Allowable stresses vs nominal strengths In the AASHTO STD LTS Specifications allowable stresses are defined in (1) bending, (2) tension/compression and (3) shear stresses. On the other hand, in the AASHTO LRFD LTS Specifications nominal strengths are defined in (1) flexure, (2) tension/compression, and (3) shear/torsion. Their expressions are drastically different. To demonstrate the differences, expressions for tube sections are listed below and their effects are studied. a) AASHTO STD LTS Specifications a1) Local buckling a2) Allowable bending stress a3) Tension/compression a4) Shear b) AASHTO LRFD LTS Specifications (for tube section only) b1) Local buckling b2) Nominal bending strength b3) Tension/Compression Strength b4) Shear/Torsion Subtask Interaction equations In the AASHTO STD LTS Specifications check is based on the combined stress ratio while in the AASHTO LRFD LTS Specifications check is based on the combined force interaction equation. Their checking based on Strength I Limit State is calibrated for samples of Maryland Standards. a) AASHTO STD LTS Combined Stress Ratio equation a1) Vertical cantilever pole type support a2) Other members: b) AASHTO LRFD LTS Combined Force interaction equation Task 6: SABRE System review SABRE was developed and maintained by this research team since early 90. In order to cast a vote of confident on the program, a complete review and conformance test on SABRE program are performed. SABRE design models of highway sign, high post, and traffic signal post are verified by alternate commercial licensed finite element analysis software, i.e. STADD Pro.

8 Task 7 Summary and Report A summary of all six tasks listed above and conclusion are included in this chapter. The report also summarizes the calibration of the AASHTO ASD and LRFD for Maryland sign and high mast light pole structure design. The enhanced SABRE program with LRFD is also addressed. Development is handled and funded by the BEST Center while the verification is be covered by this project. Training is planned and included.

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