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Reinforced Concrete Frames with
Captive Columns
Example Writing Project from:
CE 764 – Advanced Design of Reinforced Concrete Structures
Fall Semester of 2014
2
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]
3
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.
4
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]
5
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]
6
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]
7
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]
8
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]
9
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]
10
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.
11
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