Design of Girder Span Bridge with PSC Sections

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A girder bridge is one of the most common types of bridge design. Prestressing a composite girder in the negative moment region increases its stiffness by preventing cracking of concrete under service loads. In this webinar, we share the design practices of girder span bridges in Sarawak and show which functions of midas Civil can be utilized for the effective modeling of this type of structure. 

This case study highlights

1. Design Consideration of Girder Span Bridge

While designing a bridge, structural engineers often make numerous decisions that critically affect the project. In this webinar, Ir. Chong Wee Lin shares what he considers in the girder span bridge project based on past experiences.

2. Facilitate Bridge Modeling

Setting up an analytical model is very time-consuming and tedious. Often, it requires deep understanding of finite element methods. The speaker reveals to us how to shorten the whole design process by building an analytical model in a faster and easier way.

3. Application of Traffic Live Loads

Traffic loads fall under the category of static analysis but require additional knowledge. In order to apply it in practice, you need to know how to deal with various factors. Lin shares how traffic loads can be applied under different conditions.

1. Design Consideration of Girder Span Bridge

The bridge covered in this webinar belongs to the class R5 Single Carriageway and consists of RC beams, PSC I-girders, and PSC T-girders. Especially, “Link-slab” is provided to omit expansion joint over piers.

There are various types of PSC concrete grade, but commonly G50 is used, and concrete of deck slab also uses G50 to simplify the design and improve durability. 

Common choices of wire strand are 12.9mm dia, supergrade 7-wire with 15.24mm dia, and Grade 270 with 7-wire. Typical prestressing design assumptions are as follows:

- 75% jacking
- Modulus of Elasticity = 195,000 Mpa

- Anchorage draw-in = 6 mm
- Coefficient of friction = 0.20 ~ 0.25
- Wobble factor = 0.002 ~ 0.0033 per meter

The substructure consists of two RC piers in the middle and RC abutments at both ends, and pile caps(which is a rigid thick section) must be modeled to connect the piers and piles.

Figure 2. Substructure section view

Utilization of Prestressed Composite Girder Bridge Wizard

To model PSC girder bridges easily and quickly, midas Civil provides a bridge wizard function. If we define only the sections and properties of a bridge in advance, the entire bridge can be generated at once through simple numerical inputs. It provides the most necessary factors needed for modeling bridges, such as the value of elastic link according to the bearing type, the tendons' batch, and the activation date of the load/element by each construction stage.

Figure 5. Bridge wizard - Construction Stage

2. Modification of modeling from Wizard

The completed model from the wizard can be analyzed independently, but sometimes it is necessary to make some modifications to match the actual bridge. Rigid links were added at the bottom of the pier to simulate the pile cap. They modified the days, notional size, and long-term properties of the composite section to reflect the time-dependent material properties in the construction stage analysis. Rather than modeling it from an empty state, it is much more convenient to look through the wizard-generated model and make appropriate modifications.

Figure 7. Composite section for construction stage

3. Application of Traffic Live Loads

The design standards for traffic live loads are UK Department of Transport BD 37/01 and JKR Specification. HA, HB, LTAL loads are combined in accordance with design standards, and we should use different load combinations to meet the design requirements.

Midas Civil provides moving load functions to design bridges by automatically combining HA and HB to each lane and automatically reflecting the load factor based on the design standard.

Under BD 37/01 6.4.2, type HA and HB shall be combined and applied as follows:

(a) Type HA loading shall be applied to the notional lanes of the carriageway in accordance with 6.4.1, modified as given in (b) below. 

(b) Type HB Loading shall occupy any transverse position on the carriageway, either wholly within one notional lane or straddling two or more notional lanes.

Figure 9. Straddling

Midas Civil provides the ability to simulate the straddling of two lanes of HB loading. As shown in the figure below, after creating a moving load case named HA & HB (Auto), straddling lanes can be assigned. Then, the analysis results can automatically consider the case where the HB load straddles the lane of HA.

Watch the full webinar video