Introduction:
Girder spans are structural elements used to support loads over long distances, such as in bridge construction. Choosing the correct type of bearing for girder spans is crucial to ensure the stability, durability, and safety of the structure. In the case of girder spans extending to 20 meters long, deep bearings are the most suitable option.

Explanation:
Deep bearings are specifically designed to accommodate larger spans and provide increased support and stability. Let's explore the reasons why deep bearings are the most suitable choice for girder spans extending to 20 meters long:

1. Load Capacity:
- Deep bearings are capable of supporting higher loads compared to shallow bearings. This is essential for longer spans, as they experience greater forces and moments due to the weight and movement of the girder and the loads it carries.
- The load capacity of deep bearings allows for the safe and efficient transfer of the loads to the bridge piers or abutments, ensuring the structural integrity of the girder span.

2. Flexibility:
- Deep bearings offer more flexibility and movement compared to shallow bearings. This flexibility allows the girder span to accommodate thermal expansion, contraction, and dynamic loads without causing excessive stress or damage to the structure.
- The flexibility of deep bearings helps to distribute the loads more uniformly along the length of the girder span, reducing the risk of localized stress concentrations.

3. Stability and Durability:
- Deep bearings provide better stability and resistance to settlement or rotation of the girder span.
- The increased depth of the bearings helps to minimize the risk of differential settlement, which can lead to uneven loading and potential structural failure.
- Deep bearings are designed to withstand long-term exposure to environmental factors such as moisture, temperature variations, and vibration, ensuring the durability and longevity of the girder span.

Conclusion:
For girder spans extending to 20 meters long, deep bearings are the most suitable option due to their higher load capacity, flexibility, stability, and durability. Choosing the correct type of bearing is crucial in ensuring the safe and efficient functioning of the girder span, and deep bearings fulfill these requirements effectively.