Reinforced Concrete Frames with

Captive Columns

Example Writing Project from:

CE 764 – Advanced Design of Reinforced Concrete Structures

Fall Semester of 2014

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1.0 Introduction

Many buildings constructed with reinforced concrete frames use masonry or non-

structural infill as walls in between the columns. Reasons for the masonry infill range

from architectural to functional. An example might be when an existing building

undergoes remodeling and brick walls are spanned between columns. When this non-

structural material does not occupy the entire frame height, short or “captive” columns

are left to resist lateral forces. Their increased stiffness makes them a prime target for

shear failures. This paper will begin by describing what captive-columns are, give some

examples, show why the failures occur and offer potential solutions.

2.0 What are captive columns?

A very common case demonstrating the captive-column effect is when an existing

building is renovated such that non-structural elements are added in between reinforced

concrete columns, Figure 1. The non-structural elements, such as masonry or brick, have

an unintended stiffening effect on the neighboring columns. Hence, the columns are kept

‘captive’ and lateral deformation is restricted.

Importantly, the non-structural elements do not extend the full height of the column to

allow for, perhaps, a window or ventilation system. This creates an artificially short story

to carry the increased shear demands. During lateral loading, these columns are

Figure 1 – Addition of masonry in between columns. Ref. [2]

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responsible for accepting the deformation that the full height of the column was designed

to sustain [2].

3.0 Examples of Captive-Columns and Resulting Failures.

In addition to the renovation of structures that result in this configuration, many

schools and buildings built before modern seismic code requirements are victims of this

phenomenon [3]. A good example is the Valentin Valiente elementary school in Cariaco,

Venezuela. (Figure 2a) After a 1997 earthquake, several columns failed due to the shear

demands on both floors from the restriction induced by the surrounding masonry

(indicated by the red arrows). It should be noted that the column on the far left (indicated

by the orange arrow) failed in a different manner, as it was free to deform without the

masonry confinement present.

Figure 2a & b – Valentin Valiente School before and after the Cariaco, Venezuela earthquake in 1997. Ref [2] & Ref [5]

Figure 3 shows a close-up of this failure. As indicated, the columns completely failed and

shifted off their original axis causing a collapse of the original window bay and roof

system.

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Another example could be a newly constructed building in an area or country

where seismic design codes are not as recent or sophisticated as those in the United

States. Figure 4 is from a case study done in Pakistan of a building that was built after the

2005 Kashmir earthquake [7]. If the building existed during the earthquake it would have

had vulnerabilities in the circled areas. The study is part of an initiative to educate other

countries on how to identify potential seismic deficiencies and design more seismically

sound structures in the future. Incidentally, the result of the study determined that the

columns were not sufficiently reinforced and the partial masonry walls should be

removed and replaced with windows like the floors above.

Figure 4 - Potential Seismic Vulnerabilities of a Building in Muzaffarabad, Pakistan. Ref [7]

Figure 3 - Close-up of the captive-column failure at Valentin Valiente School. Ref [2]

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A final example (Figure 5) is that of a structure built on the side of a hill or slope,

where the columns supporting the floors have varying heights. Like the masonry infill in

the previous examples, the soil or engineered

backfill of a foundation system can create a

captive-column effect. As this structure

deflects, the partially buried columns would

have a higher shear demand, requiring more

transverse steel in the section.

Figure 6 shows a building collapse caused by

failure of the shortest first story columns in a

sloping terrain. The failure occurred after a 1983

earthquake in Popayan, Columbia [2].

4.0 Structural Analysis of Captive-Columns.

One might initially conclude that adding material

around a column would only make it stronger. After all, it

is making it stiffer and thicker. Shouldn’t that increase the

strength? The answer to that can be found using simple

statics. As lateral load, such as an earthquake, is applied to

a structure, the upper end of a column displaces

horizontally in relation to the bottom (Figure 7). The lateral load, along with the weight

of the structure, induces internal moment, shear and axial forces in the columns.

Figure 7 - Behavior of a Building Under Earthquake Loading. Ref [2]

Figure 5 - Structure on Sloping Ground. Ref [8]

Figure 6 - Short Column Failure on Sloping Ground. Ref [2]

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If we isolate one of these columns and analyze the internal forces, we can see how

the unsupported length of the column affects the shear demand.

Figure 8 is a free body diagram of an isolated column with the

induced forces. Summing the forces around the base, we get the

shear demand for the column, 𝑉𝑒1 = !"#$!!"#$!"

. If non-

structural material is added next to the column, this reduces the

unsupported length. Say, for example, the masonry wall is 75%

the story height, making the unsupported length 0.25*lu. The shear demand is

now  𝑉𝑒2 = !"#$!!"#$!.!"∗!"

. Therefore, 𝑉𝑒2 = 4 ∗ 𝑉𝑒1,  showing that a column originally

designed for the shear, Ve1, and later altered to a captive-column situation would most

certainly fail under moderate lateral loading.

The moment, Mpr, is the probable moment

capacity of the column [6]. Mpr is based

on the tensile steel stress of 1.25𝑓𝑦 where

𝑓𝑦 is the yield strength of the steel. The

1.25 factor is to account for some strain

hardening of the steel as the column begins

to fail. ACI 318-11 [1] and other modern day building codes account for the higher shear

demands during earthquake loading. The shear/transverse reinforcement requirements are

significantly higher than non-seismic design. With more transverse reinforcement, it

provides more confinement for the concrete and reduces intermediate buckling of the

longitudinal rebar as the column deflects back and forth. Also, this confinement is

Figure 9 - An illustration of how shortening the unsupported length of the column increases shear demand. Ref [8]

Figure 8 - FBD of a Column. Ref [6]

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important for keeping the column core intact in order to transfer the axial load down the

intended load path.

5.0 How to Mitigate or Prevent Captive-Column Failures.

Without completely removing the infill that is causing the added stiffness to the

columns, improving seismic resistance by strengthening an existing RC captive-column

is a considerable engineering challenge. It can be invasive and expensive. The following

are potential remedies to captive-column situations:

1. Provide a gap between the masonry and RC column. This gap frees the column

and allows it to deform along its full length, hence reducing the shear demands.

The gap can be filled with compressible material such as caulk, expandable foam,

a low strength grout or some material that allows the column to displace.

2. Use of Glass Fiber Reinforced Polymer (GFRP) wrap. A 2012 study [3] using

GFRP with captive-columns showed that this can be a viable solution. Its

potentially low installation cost, weight to strength ratio and good fatigue

performance support that. Figure 10 shows an example of how the GFRP is

applied. Its most important function is supplying the additional confinement that

stirrups are lacking in high shear areas. The shaded area represents the GFRP

wrap. The dimensions are those from the test

specimen used, but the proportions are such

that the beams are wrapped a distance of 2

beam depths from the column face and the

columns are wrapped to the development Figure 10 - Retrofit Scheme with GFRP Wrap. Ref [3]

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length of longitudinal reinforcement [3]. The study consisted of testing two

identical captive-column specimens except one had GFRP wrap applied. Figure

11 (a) & (b) show Base Shear vs. Deflection results from the test. Figure (a) is the

frame without the wrap and (b) is with the wrap.

From these results, it is apparent the wrap allowed for a higher lateral loading

with less deflection.

The GFRP wrapped frame provided adequate confinement for the lateral loading

such that the partial masonry infill failed before the column, indicated by the

orange arrow in Figure 12. It was

determined from this study that the GFRP

wrap can be used to upgrade captive-

columns to meet modern seismic code

requirements. The downside to this

solution is the column needs to be

completely wrapped in the GFRP to be effective. This would require some

removal (and replacement) of the existing infill.

3. Extend part of the infill the entire height of column. A study in 1995 at the

University of Andes [4] showed that simply ‘defending’ the column with some of

the infill material can support some of the critical shear. This study used non-

Figure 11 (a) & (b) - Base Shear vs. Deflection of the 2 Specimens. Ref [3]

Figure 12 - Retrofitted Frame at Failure. Ref [3]

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structural concrete as opposed to masonry as the infill material, but the behavior is

the same. Figure 13a shows a captive column failure from this study. Figure 13b

shows failure when some of the infill is extended the full height. Although

cracking is significant, the RC column is still intact in the column-defended

specimen. This is important as the load path mostly remains intact and helps to

prevent a total collapse.

6.0 Conclusion.

In summary, a short or captive-column is one that has a different unsupported

length than the other columns of the same story. By reducing the unsupported length of a

column, it is clear the shear it experiences is higher when the deflection remains the

same. Due to the seismic vulnerabilities captive-columns can pose on RC frames,

measures should be taken to avoid or properly reinforce them if possible using modern

seismic design codes. However, in cases where they exist or need to, it is important to

properly account for the increased shear demands placed on them from earthquake or

high lateral loading. Engineers can do that by implementing some of the remedies

discussed above. By providing a gap between the column and the stiffening material, or

wrapping the frame in a Glass Fiber Reinforced Polymer or ‘defending’ the column by

extending some of the material the full height of the story, shear stresses can be at least

Figure 13 a - Captive Column Failure. Ref [4] Figure 13 b – Failure with added infill. Ref [4]

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partially, if not fully, accounted for. Additionally, by educating other countries with less

sophisticated seismic design practices, serious or total structural failures can be avoided

in the future.

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References: [1] ACI (American Concrete Institute). (2011). “Building Code Requirements for Structural Concrete.” ACI 318-11, Farmington Hills, MI. [2] Guevara, T. L. & Garcia, L. E. (2005). “The Captive- and Short-Column Effects.” Journal of the Earthquake Engineering Research Institute, Vol. 21 [3] Jayaguru, C. & Subramanian, K. (2012). “Retrofit of RC Frames with Captive-Column Defects.” KSCE Journal of Civil Engineers, 10.1007/s12205-012-1019-5 [4] Pineda, J.C., (1995), “Experimental Tests on the Control, of Short Columns.” Senior Project IC-94-II-26 Advisor: LE Garcia, Department of Civil Engineering, University of the Andes at Bogota, Columbia [5] Lopez, O.A., Marinilli, G., Coronel, G. & Bonilla, R. (2012) “Improving Seismic Safety in Venezuelan Schools.” FUNVISIS, Ministry of Science and Technology, Venezuela [6] Nilson, A. H., Darwin, D., and Dolan C. W. (2010). Design of Concrete Structures, Wiley, 14th Edition, Chapter 20, McGraw-Hill, NY [7] Khan, K., NED University of Engineering and Technology, & Rodgers, J., GeoHazards International. Report: “Four Storey Office Building in Muzaffarabad: A Case Study of Seismic Assessment and Retrofit Design.” [8] Ramin, K. & Mehrabpour, F., (2014), “Study of Short Column Behavior Originated from the Level Difference on Sloping Lots During Earthquake (Special Case: Reinforced Concrete Buildings.” Open Journal of Civil Engineers