UCII: On-Line Bridge Monitor

AUTOMATED TRAFFIC MONITOR CONDITIONS AND/OR ASSUMPTIONS:

The follow is a listing of the known conditions and/or assumptions used in the development of the HAM-126-0881L automated Traffic Monitor and the associated automatic rating calculations. The various conditions and assumptions are sorted here into various categories. Some of these conditions can and will be eased as more sophisticated hardware/software is added to the site allowing for more information processing. Others are inherent limitations of the instrumentation configuration or the structural configuration of the bridge and its design details. Still others are standard assumptions used by structural engineers and incorporated into the AASHTO specifications and rating calculations guidelines.

In addition to the points noted here, this document provides a discussion of the general behavior of bridges with integral abutments in response to transient, permanent, and environmental loads.

Derivation of the Influence Line and HS20-44 Truck Response:

1. Truck maintains an approximately constant speed: if the truck rapidly slows or accelerates, this will lead to errors in the algorithm. A 10% variance causes approximately 3% error.

2. Truck speed is 60mph or less: A faster speed will clip our filtered bridge response and lead to underestimation of the influence line and overestimation of the ratings.

3. The truck consists of 5 axles or less. At present, only the data on the first 5 axles is obtained from the WIM.

4. No other truck traffic upon the bridge: this is less of a concern for truck rating (unless the trucks are side-by-side), but will lead to great overestimation of the laneload rating.

5. Truck remains within the boundaries of the lane: if the truck changes lanes, then the rating will be overestimated because the bridge response is greater from a truck in the right lane.

6. Truck does not have a loose or bouncy suspension (e.g., semi versus dump trucks): this causes low frequency noise below our filter frequency (3.5 Hz) that will generally lead to overestimation of the ratings (e.g., a controlled 50mph test with a semi-truck had 4% error).

7. Truck is fully loaded to minimize bounce: again, this causes low frequency noise below our filter frequency that will generally lead to overestimation of the ratings (e.g., a controlled 50mph test with a lightly and fully loaded truck resulted in 7% and no error, respectively).

8. Dynamics of the structure have been removed: A lowpass filter is included within the real-time processing of the data to remove the dynamic response of the structure itself, as determined from modal vibration experiments (e.g., first mode identified at 4.5 Hz). It must be noted that this can only be realized by crawl-speed tests (e.g., 10mph) of this particular structure due to its very low span ratio (0.45); however, the error is made small by filtering.

9. The Weigh-in-Motion (WIM) system is maintained within tolerances: the accuracy of our bridge monitor depends directly upon the accuracy of the WIM.

10. Linear Superposition: the deformation for the simultaneous application of two or more loads is equal to the sum of their respective deformations.

11. Small strains and deformations: geometry changes are negligible and slopes are very small.

Derivation of the AASHTO Truck and Lane Load Ratings:

1. The safety factors used in AASHTO load ratings constrain the analysis to the linear range.

2. A series of controlled truckload experiments has shown that the most critical location for this particular structure is the middle beam at midspan of the middle span. This is due to the very low span ratio of this bridge. The sensor data and ratings displayed are for this location. Note that truckload rating controls this location; the laneload rating will be lower at the piers.

3. The load rating analysis is performed for inventory and operating loading by AASHTO HS20-44 specifications. A rear axle spacing of 14 feet for the truckload will yield the lowest rating for a positive moment region, so it is employed here. The rating analysis methods of Allowable Stress and Load Factor Design (LFD) are used.

4. Material capacities (e.g., yield force) are given either by the bridge plans or by AASHTO, dependent upon the year of construction. For this bridge, fy = 50 ksi and f’c = 4.5 ksi.

5. An inch deep wearing surface is assumed for the deck. Effective deck width (93 inches) is determined as twelve times the deck thickness (8.75 inches) less the wearing surface. This assumption causes the structure to fail the compactness check for its composite section. Hence, the ultimate moment is reduced to the yield moment and the ratings are similar by either method employed. If the wearing surface is included within the deck thickness, then the LFD ratings will increase by approximately 50% for the critical section being monitored.

6. Deadload is determined by finite element model, using pins at the integral abutments and rollers at the piers. Composite action is modeled as designed by using rigid links to connect the deck and girders in the middle span. Composite action in the middle span is verified and monitored by embedded deck sensors. The deadload moment is 6540 kip-inch for the critical section being monitored.

7. Superimposed deadload is simulated as a uniform load to be shared equally by all girders. The concrete strength is reduced by a factor of three to account for creep. The superimposed deadload moment is 2550 kip-inch for the critical section being monitored.

8. Deadload forces were measured during construction, but not included within these rating equations. The deadload and superimposed deadload moments (5971 kip-inch and 435 kip-inch, respectively) were found to be lower than the design values (6540 and 2550) for the critical section being monitored; if included, the ratings would higher.

9. Dynamic effects are accounted for by AASHTO within the impact factor, I = 50/(L+125). For the critical section being monitored, the impact factor is I = 50/(88+125) = 0.235. If desired by the owner, the impact factor could be calculated from a statistical sample of the actual bridge traffic. As the formula is conservative, this would increase the load rating.

10. The number and lanes of traffic under consideration shall be the actual marked travel lanes, as specified by ODOT. This particular structure has two marked lanes, so there are no reduction factors applied for multiple lanes.

11. Sectional capacities are analyzed as to the intended method of design. Hence, unintended composite action between the concrete deck and steel girder is expected to be lost at the limit state. For this particular structure, the critical section being monitored was designed to act (and continues to act) as a fully composite section.

12. Unintended bearing restraint will remain unchanged throughout the linear load range: if the owner wants to consider loss of bearing restraint, then additional analysis must be performed and the load rating will be modified accordingly (i.e., increased rating at supports and decreased rating at midspans). Note that this particular structure has integral abutments with significant rotational restraint and elastomeric pier bearings with axial restraint and neither act as assumed by design.

13. Axial force is calculated, but not included within these rating equations.

14. Temperature forces are regularly measured, but not included within these rating equations. The forces can be quite large compared to liveload values (e.g., 6 ksi for the critical section being monitored), but they occur annually. AASHTO allows for thermal forces as an additional superimposed load in reducing the Operating rating.

15. Bernoulli Theorem of Bending: plane sections will remain plane after bending.

16. Homogeneity: beam material is consistent throughout the cross section.

17. Hooke’s Law: beam material has a linear stress-strain relationship as defined by the modulus of elasticity, E, and it is the same in both tension and compression. For this particular bridge, E = 29,000 ksi for steel, E = 3,600 ksi for concrete, and their ratio is 8.

18. Vertical symmetry: to ensure that the stress distribution is symmetric, that the y axis is the principal axis, the shear center will lie on the y axis, and bending without twisting will occur.

19. Straight beam design: although it can be extended to curved beams if the ratio of depth to radius of undeformed curvature is small.

20. Prismatic beam design: beam is long, slender, and has a constant cross section, although the latter can be relaxed if the change in cross section is gradual and continuous.

21. Stability under bending: cross section is to maintain integrity and shape under bending action (which may not occur for thin-walled elements).

22. One dimension: all deformation is to occur in the x-y plane (i.e., no lateral or torsional behavior).