Quantification of Compressive Membrane Action in Reinforced Concrete Bridge Decks

Author: Yannick Kummer
Language: English

Abstract

Many existing bridges in Switzerland do not satisfy fatigue checks due to increased loads and conservative design. The absence of damage suggests structural reserves that need to be exploited to avoid costly measures. One method is to consider Compressive Membrane Action (CMA). By restraining the bridge decks against crack-induced expansion, an arched load transfer is formed and reinforcement stresses are reduced (Fig. 1).

Traditionally, research focused on transverse CMA at the ultimate limit state, neglecting the longitudinal direction. This thesis examines the central section of a box girder bridge and investigates the effects of local transverse and longitudinal CMA combined with the significant longitudinal global membrane forces (GMF) resulting from vertical loads and prestressing.

To better understand the load-bearing behaviour of partially restrained slabs, a simplified line-supported model was created, for which a uniaxial fatigue load was applied to the centre of the slab. This model simplifies interpretation and eliminates membrane forces (GMF) from bending.

Fig. 2 shows that transverse cracking at 0.2x fatigue load leads to increased membrane forces in Fig. 3 for the central element of the slab (CMA effect). In the longitudinal direction, a higher GMF results in a longer fully compressed state Fig. 2, thereby further reducing the reinforcement stresses.

Fig.1: Arched load transfer for restrained slabs [1]

Applying the load to the mid-section of the three-span box girder bridge results in both GMF and local CMA. This results in a more complex isolation of the local CMA forces, which has been solved by fitting a surface to the distribution of membrane forces (Fig. 4).

The simplified model helps to interpret the results of the more complex behaviour of the bridge deck. It is assumed that the webs and the tension ring lead to a concentration of the transverse CMA forces. For instance, it shows that inclined webs increases the CMA in the cantilever slab, which reduces the reinforcement stresses over the web.

This thesis successfully demonstrates the potential of CMA to reduce the reinforcement stresses by using a more refined analysis, particularly when combined with longitudinal GMF. Future work should extend the calculations to bridge sections with lower or tensile GMF and other bridge types. Experimental validation is required to pave the way for the systematic integration of CMA into the assessment of existing structures.

[1] Thoma et al. (2019). Fatigue strength of deck slabs loaded predominantly in bending, AGB Project 2010/001

Fig. 4: Isolation of CMA from overall membrane forces

Fig. 5: Principal CMA for inclined vs. vertical webs