Reasons and Solutions for Transverse Sweep Occurrence in PSC Girders

Ⅰ. Introduction

PSC girder bridge engineers are likely to be familiar with the lateral sweep of PSC girders.

The lateral sweep of PSC beams has become more common in recent years due to the following factors:

• With the advancement in construction technology, the length of the girder has increased to more than 50 meters,

• The number of strands installed in the girder has also increased,

• As the girder height increases, it becomes stiffer in the strong axis direction, but its transverse stiffness becomes weaker.

However, lateral sweep is still not discussed in detail or given clear countermeasures.

At a bridge construction site where I was responsible for an engineering consultant, lateral sweep of PSC girders was found. during that time, I had to do a lot of research and collect data to solve the problem.

In this content, I will discuss the data I collected to solve the lateral sweep of PSC beam.

Ⅱ. Guidelines and Examples for Lateral(Horizontal sweep)

2.1 Guidelines for lateral sweep

The criteria for lateral sweep may be a new concept for many engineers because it is not clearly specified in the design criteria.

If you ask an engineer or design company, "What should we do if the girder is bent?", most of them will answer, "Dispose of it and make it again.", since it has been designed with the assumption that the girder will not bend.

However, in the field, it is not easy to make such a decision. In such cases, the field engineer has no choice but to ask or force the design engineer to solve it.

Since the girders used in bridges are basically designed to resist longitudinal loads, their stiffness is relatively small in the lateral direction.

So, let's take a look at what the regulatory guidance says about lateral sweep tolorence.

In general, Korea applies the PCI manual and sets L/1000 as the lateral sweep tolerance limit.

Korea Expressway Corporation suggests a sweep tolerance of L/1000 with a design example.

2.2 Examples of laterally swept beams

I would like to show examples of lateral sweep that I have experienced in the field. These examples include photos of beams that were under construction and beams that have been completed on bridges.

Note: The project name & details are not mentioned

2.2.1 Examples of laterally swept beams that were found before the deck slab was poured

 

 

Example 1	Example 2
Example 3	Example 4

 

2.2.2 Examples of laterally swept beams that were found after the bridge was completed

 

 

Example 1	Example 2

 

Ⅲ. Causes of laterally swept beams

There are many causes of lateral sweep in PSC beams.

As one of our technical advisors said

Lateral sweep of PSC girders is not only common, but it can also be caused by unexpected things.

The following are some of the most common causes of sweep.

① Lateral sweep during the PSC prestressing process

PSC beams are typically designed with symmetric tendons, so lateral sweep will not occur if the beam is manufactured as designed.

However, in practice, lateral sweep can occur due to a number of factors, such as

• assembly errors in the sheathing pipe,

• elastic deformation of the beam due to the order of tensioning,

• and breakage of the pre-tensioned strands during tensioning.

 

 

Application of prestressing

 

* Even if the tendons are symmetrically arranged, the tension is applied in sequence.

② Lateral sweep caused by direct sunlight

The most unexpected cause of lateral sweep that I have learned from a technical consultant during a consultation, is direct sunlight.

If one side of a beam is exposed to direct sunlight, it can cause a temperature gradient in the concrete. This temperature gradient can be the cause of lateral sweep.

I have confirmed with the field engineers around me that there are many cases like this.

③ Lateral sweep caused by formwork deformation during production

This is a scenario where lateral sweep occurs due to errors in the installation of the girder fabrication unit.

This is an item that should have a low occurrence rate if there is a proper site inspection or quality control team.

④ Lateral sweep caused by a change in the center of gravity during beam installation

This is probably the most unwanted cause of lateral sweep in a construction site.

PSC girders are basically designed with a tendons that is capable of resisting longitudinal loads. However, during the erection process, the direction of action of gravity that should be acting in the vertical direction may deviate.

In this case, the compressive force from the tendons and the moment generated by the eccentricity of the tendons will be different from the design, and this will cause the existing lateral sweep to be worse or new lateral sweep to occur.

⑤ Lateral sweep due to material non-uniformity

Concrete is a mixture of different materials, and homogeneous quality cannot always be achieved.

Lateral sweep can occur due to concrete admixtures or aggregate deformation that are not considered in the design.

⑥ Lateral sweep due to support conditions such as uneven settlement of the fabrication plant

If there is a problem with the support conditions of the member where the tension force is applied, the eccentric effect of the tension force will occur, and it will become one of the main causes of lateral sweep.

There was a case where lateral sweep occurred in the entire girder installed at the same span on site, and to identify the cause, we checked the girder manufacturing site with Google Maps.

We could see that there was a problem with the site ground condition at the fabrication site.

 

 Girder fabrication site

 

⑦ Additional deformation due to concrete creep (long-term behaviour)

Creep refers to the phenomenon that deformation continues to occur over time even though no additional load is applied to the concrete, and it is a factor that leads to the worsening of lateral sweep.

Once the initial sweep has occurred, the PSC girder will experience an increased in transverse displacement even if no additional load is applied.

Ⅳ. Methods for handling lateral sweep beams

I analyzed various cases while providing technical support on site, submitted a review report based on the results of the analysis. However, as these contents are not explicitly mentioned in the design standards, please check them only as reference materials.

4.1 Solutions before deck slab casting

Most lateral sweep is discovered in the production plant or before deck slab casting, so deck slab is cast after the correction work is completed.

4.1.1 Impact Analysis method

As mentioned in Chapter 3, once a lateral sweep has been occured, further deformation occurs due to creep deformation.

Therefore, the creep deformation coefficient is calculated based on the concrete age at the time of measuring the sweep and the time of introducing PS tension force, and the elastic deformation amount is estimated from the total deformation amount.

① Calculation of creep coefficient (Refer to Highway bridge limit state design method 5.5.1.4)

Here,

② Calculation of elastic strain

The creep coefficient can be defined as,

and total displacement can be calculated as "total strain = elastic strain + creep strain".

③ Stress calculation due to lateral sweep

The elastic deformation is calculated from the total deformation, and based on the displacement the end moment or lateral sweep that causes elastic displacement can be calculated.

In other words, the additional stress caused by the elastic displacement can be calculated.

The additional stress generated by the lateral sweep is added to the design stress to check whether it is within the permissible limit.

4.1.2 Countermeasures

Among the methods for correcting the deflection caused by lateral sweep, which have been investigated in the field, there are two types: pulling and pushing, and the work order is as follows.

 

 

Classification	PULLING	PUSHING
Sequence of operation	1. Horizontal beam welding at end after girder setting 2. Wire rope winding (at support and location of sweep)3. Chain block connection 4. Apply puling force 5. Check the amount of correction (dial gauge) 6. After calibration, install transverse support wood on the top flange of the girder 7. Cross beams welding	1. Cross beam welding at end after girder setting 2. Install hydraulic jack/screw jack (between support girder and compensating girder) 3. Apply pushing force 4. Check the calibration amount (dial gauge) 5. Install transverse support wood on the top flange of the girder after completion of calibration 6. Cross beam welding
Example

 

4.2 Countermeasure after pouring deck slab concrete

4.2.1 Impact Analysis

If a lateral sweep is discovered after the girder has been poured up to the deck slab, it is very likely that the construction was not properly inspected.

After the completion of the curing process for the deck slab and crossbeam concrete, if there was sufficient transverse curvature to be visibly detected, it would indicate significant structural issues not only with the girders but also with the deck slab.

However, in this condition, it is not possible to correct the sweep of the girders mentioned in Section 4.1.

Replacing the beams of a completed bridge is a very complex process, so construction engineers would like to prove that the structure is safe even if lateral sweep occurs.

However, the assessment to prove this must be done very carefully.

4.2.2 Countermeasures

① The elastic strain can be predicted using the method mentioned in 4.1.1, and the additional stress can be calculated and added to the design stress to evaluate the safety of the girder.

② If the sweep is not large, a structural review can be performed by considering the change in section stiffness due to the axial change of the main girder and the change in load due to the lateral sweep.

*In the case of a girder resting on an elastic base, girder rotation may occur due to the difference in center of gravity due to lateral sweep.

Note: If the amount of lateral sweep affects the safety of the beam, bracing can be added between the beams to prepare for lateral sweep.

Ⅴ. Conclusion

The phenomenon of lateral sweep of beams is probably not a topic that practitioners of bridge design engineering have given much thought to.

This is because it is considered a construction error and something that the contractor should take care of.

However, since lateral sweep is an unavoidable phenomenon in most PSC girders, I thought it would be a good idea for engineers to consider it while designing.

If you have experience in analyzing and correcting sweep in other ways, please do share your experience.

I believe that sharing experiences and knowledge will help engineers grow.

Reference

[1] Explanation of Road Bridge Design Criteria (Limit State Design Method) (2015), Korea Institute of Bridge and Structural Engineers

[2] Highway Design Practice Guidelines (2021), Korea Expressway Corporation

Topic

Girder Lateral Sweep

PSC Girders

Lateral Sweep Design Criteria

Lateral Sweep Examples

Causes of Lateral Sweep

About the Editor

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Structure & Architecture engineer Over 20 years of experiences

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