Typical Bridge Load test to evaluate the load carrying capacity of bridges

 1. INTRODUCTION

Evaluation test for load carrying capacity of bridge will be useful to assess the effects of defects or deterioration on the overall performance of the bridge or on the stresses in individual components. This note presents a procedure for assessing the safe load carrying capacity of single span PSC box girder Major Bridge (as per drawing). In view of the safety of spans to service loads it is recommended to assess the safe load carrying capacity of the bridge. Load test is planned according to the guidelines given in IRC SP:51“Guidelines for evaluation of load carrying capacity of bridges”.

2. TYPICAL LOADING ARRANGEMENT

The test load was applied using Hyva & sandbags, in the form of static loads on span imprints of the specific class of position on span. The Hyva & sand bags were positioned at the pre-determined locations which produces the critical bending moment as per the below drawing.

3. INSTRUMENTS FOR MONITORING

Deflection and temperature were measured using following instruments and the measurements were recorded in the Data logger. The instrument details make and model will be furnished independent engineer.

LVDT (Linear Variable Differential Transformer) for Deflection Measurement

Thermometer (for Temperature measurements)

4. METHODOLOGY FOR TESTING :

a. All the visual defect has been checked prior to conduct the test. Expansion joints and bearings shall be ensured for their functional condition.
b. Scaffolding shall be erected for fixing the deflection measuring gauges on their pre-defined most suitable position by which those gauges are able to determine the necessary out comes of deflections and separate scaffolding for making observation. All scaffolding shall be stable and should be safe from any outside force or movement.
LVDT

LVDT Deflection Test

c. Initial readings (R1) of LVDTs were noted down before placing the test load and sensors were fixed as per the drawing locations.
d. After that deflection along with ambient temperature data were collected from dawn to dusk at 1hour interval. The temperature at are recorded till completion of test.
e. The position of Hyva & sand bags were placed on the bridge span as per the drawing. Hyva & sand bags were stationed at the marked location. The Hyvas & sand bags were weighed by calibrated electronic weighing machine available at site before placing them on the span for loading purpose. Readings were noted during loading time as well as after completion of loading of all Hyva & sand bags for live/test load the deflection were measured at each of the locations of LVDT.
The (Typical) test load were applied in stages of 0% 0% , 17%(50.5T), 45%(131.5T), 63%(185.5 T), 72%(212.5T), 91%(266.5T), and 100% (293.5T)load applied of Total Test Load (293.5T).
Then the final loading (100%) was kept as it is for 24 hours and deflection measurement was taken for 24 hours at 1 hour interval with the final reading (R3) of loading. Next unloading were commenced gradually and the span was kept for 24 hours past completions of unloading the final deflection were recorded for computing the extent of recovery.
f. The deflections were measured one hour after loading up to 100% loading was completed. Each increment of loads is increased after stabilization of deflection. The recording of deflection value was continued for next 24 hours at1hour interval. (After completion of 24 hours the above procedure was repeated for unloading of load and deflection values (R5) were recorded.
The test span were constantly monitored for appearance and widening of any cracks at every stage of loading and unloading.

5. ACCEPTANCE CRITERIA FOR STRENGTH:

The percentage of recovery of deflection for ‘PSC box girder’ Girder Bridge at 24 hours after removal of test load shall be 85%.As per IRC-SP-51 (2014).

Calculation Theory

Calculation Theory
The percentage recovery shall be calculated for values of deflection. The percentage recovery is calculated at 24 hours after removal of load, the analysis is carried out as follows, after effecting temperature correction, bearing displacement correction and or rotation corrections to deflection
data:
a) Initial value – deflections before commencement of loading = R1
b) Deflections at one hour, after placement of 100 percent test load = R2
c) Deflections at 24 hours after placement of 100% test load = R3
d) Deflection measurements immediately after removal of test load = R4
e) Deflection measurements at 24 hours after removal of test load = R5
f) Total deflection = R3 – R1
g) Total recovery of deflection after 24 hours after removal of test load = R3-R5
h) Percentage recovery of deflection 24 hours after removal of test load
(Where this value exceeds 100 percent it shall be restricted to 100 percent)

Source: IRC SP 51

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