structural REPAIR
Novel Solution for Strengthening Handrail
Anchorage
By Ali Abu-Yosef, Ph.D., P.E., S.E., Joseph Klein, P.E., Michael Ahern, P.E.,
and Randall Poston, Ph.D., P.E., S.E., NAE
D
esign and detailing of handrail anchorages in concrete
structures must consider both structural performance
and susceptibility to corrosion. Without these dual considerations, handrail anchorages are more likely to fail. For
decades, premature corrosion of handrail anchorage components was prevalent in the concrete industry. Deterioration
of the anchor embedments due to galvanic corrosion, direct
exposure of aluminum to concrete, or insufficient concrete
cover resulted in costly repairs.
While improvements in structural detailing and material selection to
mitigate corrosion have been successfully implemented and are now
common practice, the structural design and installation of handrail
anchorages remains a minefield. A primary cause of continued handrail
anchorage issues is the lack of clear delineation of design responsibilities and detailed coordination between the architect, handrail system
manufacturers, the engineer of record, and the contractor. As a result,
inappropriate assumptions and poor communication remain a source
of deficient handrail installations.
Repair of structurally deficient rail-post anchors is both challenging
and costly. Rail-post anchorage repairs often must maneuver tight
geometries, a concrete substrate congested with steel reinforcement,
numerous forms of potential corrosion cells, and elevated access
limitations. This article presents a novel repair solution that was used
to address structurally deficient handrail anchorages in a high-rise
building. The repair approach presented herein uses inert materials
that are not susceptible to corrosion due to environmental exposure.
Figure 1. Free body diagram for the anchor reaction forces produced by handrail
loading. Adapted from Raths 1974.
analytical methods can be used to provide rational estimates of the
reaction forces that develop within the anchorage assembly due to
externally applied shear and moment forces. The free-body diagram
shown in Figure 1 is based on the findings reported by Raths (1974)
for steel embedments in concrete at the ultimate state. The externally
applied handrail forces are resisted by a shear couple (CF and CB) that
develops within the embedded depth of the anchorage assembly. The
free-body diagram can be used to calculate the breakout shear demand
(CF). It should be noted that Figure 1 does not include wind loading
for simplicity, but the demands due to wind loading can be included
similarly. In addition, a load reversal is possible but results in smaller
concrete breakout stresses due to the relative magnitudes of CF and
CB per the equation, (CF = VU + CB).
Handrail Anchorage Assemblies – The Basics
The International Building Code (IBC) specifies the minimum applied
live loads on balcony railings. The effects of live and wind loads need
to be considered when designing handrail components. The selected
anchorage system should resist the effects produced by a 200-pound
concentrated live load or a 50 lb/ft linear live load applied directly
at the handrail, as well as location- and elevation-specific wind loading. The applied loads are transmitted from the railing posts into the
anchorage assembly and the concrete slab.
The post anchorage should be designed to resist the effects of the
externally applied shear force and moment couple, without causing
breakout failure of the concrete slab. Calculating the force that is
developed within the anchor is not straightforward. This is particularly
true for anchorages embedded into the concrete slab. Several published
30 STRUCTURE magazine
Figure 2. Concrete removal along a slab edge exposed grout pockets with missing
hairpin anchor reinforcement.
Handrail anchorage assemblies in concrete elements are designed
following the requirements of Chapter 17 of ACI 318-19, Building
Code Requirements for Structural Concrete. According to ACI requirements, the breakout shear force can be resisted by either the shear
breakout resistance of the concrete material at the slab edge or anchor
reinforcement. Unlike one-way beam shear strength requirements,
the contribution of the concrete resistance and the anchor reinforcement cannot be added when calculating the breakout shear strength.
The concrete shear breakout strength is proportional to the distance
separating the anchorage embedment and the slab edge and is calculated using the formulas in Section 17.5.2 of ACI 318-19. In typical
balcony installations, the edge distance is minimized to increase usable
balcony space. As a result, the concrete breakout strength is marginal
and often insufficient to resist the applied design forces. Hence, anchor
reinforcement, commonly steel hairpins (U-shaped reinforcement
with legs extending back into the slab), are used to reinforce typical
anchorage assemblies in concrete balconies. Article 17.5.2.9 of ACI
318-19 permits the use of properly developed hairpin anchor reinforcement to resist the applied breakout forces. Because the concrete
breakout strength is marginal, failure to provide anchor reinforcement
around handrail posts due to improper design or installation can lead
to anchor failures. The following discussion examines a case study
of deficient handrail anchorages and the repairs performed to ensure
adequate structural performance.
A Hidden Deficiency
Figure 3. Details for typical NSM GFRP hairpin repair.
with building code requirements. Due to the limited edge distance
between the grout pocket and the slab edge, the post anchorages were
susceptible to breakout if design-level, or even service-level, loads
were applied. Hence, structural repairs to strengthen the deficient
anchorages were necessary.
A Novel Repair Approach
Given that the existing and repaired slab edges did not have sufficient breakout strength to resist the design loads, it was evident that
mechanical strengthening using post-installed reinforcement was
needed. However, the use of drilled-in, post-installed steel anchor
reinforcement was not a feasible option. To adequately develop a
post-installed anchor reinforcing bar beyond the breakout failure
plane, minimum 12-inch-deep holes had to be drilled into the slab
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Rail-post anchorage deficiencies were discovered during slab-edge
repairs on an 8-year-old high-rise condominium. The slab edge repairs
were performed to address the widespread corrosion of steel reinforcement. The premature corrosion was the result of improper placement
of reinforcing bars along the post-tensioned (PT) slab edges in the
54-story building and mostly unrelated to the rail-posts anchorages.
The repairs included removal and replacement of more than 1.5 linear
miles of slab edges directly exposed to weather. To mitigate future
corrosion problems, the repairs utilized glass-fiber-reinforced polymer
(GFRP) bars to reinforce and anchor the newly cast slab edges.
The balcony railing system at the building consists of an aluminum
frame with glass panels. The railing posts are embedded into 4-inchdeep grout pockets that were blocked out during placement of the
5.5-inch-thick PT slabs. The blockouts were later filled by no-shrink,
high-strength mortar after the railing posts were secured in place.
The original design called for No. 5 steel hairpin reinforcement to be
placed around the grout pockets with a top concrete cover of 1 inch.
The diameter of the grout pockets is approximately 4 inches, with
the centerline of the pockets located 3 to 6 inches from the slab
edge. As a result, the distance between the outer edge of the grout
pocket and the slab edge ranged between 1 and 4 inches. The rail
post-block-out position and depth contributed to congestion and
low reinforcement cover in the vicinity.
During the early stages of repair construction, the contractor
exposed railing post anchorages that did not have the No. 5 hairpin
anchor reinforcement as specified in the original design (Figure 2).
The issue of missing anchor reinforcement was only observed at
the bottom four stories of the residential tower. A review of the
original construction documents suggested that the culprit for
the missing anchor reinforcement was likely miscommunication
between the structural engineer, the railing system manufacturer,
and the contractor.
As constructed, the railing anchorages with missing anchor
reinforcement were structurally deficient and did not comply
JANUARY 2020
31
substrate. Due to steel congestion near the slab edge and the
presence of PT tendons and anchor components, the risk of
accidentally damaging existing reinforcement and PT tendons
during drilling was substantial. The steel congestion near the
slab edge mitigated the use of non-destructive testing methods
to reliably locate embedded PT components before drilling.
The repair team also considered using externally applied carbonfiber-reinforced polymer (CFRP) sheets to strengthen the slab
edges around the post anchorages. However, due to the slab
edge geometry and the presence of a drip edge along the slab
soffit, the CFRP sheets could not be adequately developed.
Furthermore, the repaired slab edges were to remain exposed,
with only a thin layer of elastomeric coating. Hence, the surface applied CFRP sheet fibers would have visibly affected the
aesthetics of the repair.
Given the numerous project restraints, the repair team determined that near-surface mounted (NSM) reinforcement was
an ideal solution. NSM reinforcing bars are embedded within
purposely prepared surface grooves using epoxy. Stresses are transmitted from the reinforcing bar to the epoxy and then to the Figure 4. GFRP hairpin installed around a railing post grout pocket.
concrete substrate through mechanical bond. Because the NSM
grooves do not extend more than 1.0-inch-deep into the slab, the the NSM reinforcement solution has little to no impact on the repair
risk of damaging existing reinforcement or rupturing PT tendons aesthetics and effectively resists the design forces.
is mitigated. The epoxy-filled NSM grooves can be leveled with the
If poorly detailed or constructed, the use of steel reinforcement in
top concrete surface, which is eventually coated with an elastomeric NSM repairs can lead to premature corrosion. To mitigate corrosion
waterproofing membrane. Also, a top surface placement is most effec- risk, the design team opted to use GFRP hairpin reinforcement for the
tive in resisting the moment resulting from applied forces. Hence, handrail anchorage repairs. GFRP bars are electrochemically inert and
are not susceptible to corrosion, regardless of the exposure conditions.
ACI 440.2R-08, Guide for the Design and Construction of Externally
Bonded FRP Systems for Strengthening Concrete Structures, provides
®
design provisions and construction guidelines for NSM systems
with FRP bars. The size of the GFRP bar is first determined by
evaluating the tensile capacity of the bar. The tensile strength of
GFRP reinforcement must be reduced to account for environmental
BETTER PERFORMANCE
exposure effects. Also, the tensile strength of GFRP reinforcement
with bends (for example, hairpins) is further reduced to account for
the stress concentrations that can occur within the bend region. ACI
Committee 440 provides guidelines for minimum allowable GFRP
bend radii to alleviate the stress concentrations at the bend locations.
The repair specifications in this project also restricted the allowable
amount of cross-sectional distortion of the bends to reduce the effect
of manufacturing discrepancies on stress concentrations.
Based on the calculated demands, the analysis determined that a
No. 3 GFRP hairpin was sufficient to provide anchor reinforcement
for each handrail post. The length of the hairpin legs was determined
based on the bond strength criteria provided in Section 13.3 of
ACI 440.2R. For the selected bar dimensions and load demands,
a development length of 12 inches was needed. The surface groove
dimensions were detailed based on the ACI 440 guidelines. The
minimum specified depth and width of the surface grooves (¾ inch)
were greater than 1.5 times the GFRP bar diameter. Also, the groove
surfaces had to be roughened and cleaned before installation of the
epoxy adhesive and the GFRP bar.
Because slab edge repairs were in progress immediately adjacent to
the rail posts, measures had to be developed to reliably incorporate
the NSM repairs into the newly cast slab edges. To this end, the
repair engineer provided two alternatives for the contractor. The first
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alternative allowed for placing a blockout over the newly cast slab
edge to form the needed NSM groove. This option was ideal if the
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32 STRUCTURE magazine
the NSM installation. The blockout is then removed, and the groove
surfaces are prepped and cleaned before the GFRP hairpin installation.
If the NSM installation could be performed before the slab edge
was cast, the contractor was allowed to embed the curved portion of
the hairpin into the repair concrete. Because the thermal expansion
properties of concrete and GFRP are different, the design specified
an increased concrete cover over the portion of the GFRP hairpin embedded in concrete and not epoxy. Shallow concrete cover
over GFRP bars can result in cracking due to a thermally-induced
strain differential between the concrete and the GFRP material. As
such, while a clear cover of ⅛ inch was sufficient for portions of
the GFRP bars covered with epoxy, a more significant cover was
needed for portions embedded in concrete. Per ACI 440 guidelines, the portions of the GFRP hairpin that were embedded in
concrete had a vertical cover of at least ½ inch. This was achieved
during construction by gradually increasing the depth of the NSM
groove as the distance to the slab edge decreased. Figure 3 (page 31)
schematically shows the repair details, and Figure 4 shows a GFRP
hairpin soon after installation.
The repair contractor was able to perform the repairs without removing the railing components, which further reduced the cost of the
repairs. The groove edges were created using shallow saw cuts, and
then 15-pound chipping hammers were used to remove the concrete
within the sawcut boundaries. The groove surfaces were roughened
per ACI 440 requirements to improve the bond between the epoxy
and concrete substrate. The epoxy adhesive selected for the NSM
repair was chosen based on reported past performance and recommendations of the GFRP bar manufacturer. During NSM repair
execution, the railing system in each of the affected balconies was
temporarily supported in the out-of-plane direction, and public access
to balconies was restricted to prevent failure of the handrail system
during repair execution.
Conclusion
The use of fiber-reinforced-polymer bars in concrete construction has
increased rapidly over the last few decades. Due to their inherently
inert characteristics, GFRP bars provide an attractive solution for
structures in corrosive environments. As demonstrated here, GFRP
reinforcing bars are a viable solution for use in repair projects. The
limitations and challenges encountered in this project were not unique
or isolated, and this repair option provided an effective means
of avoiding the traditional pitfalls of conventional rail post
anchorage repairs.■
The online version of this article contains references.
Please visit www.STRUCTUREmag.org.
All authors are employed by Pivot Engineers, PLLC in Austin, Texas.
Ali Abu-Yosef is a Senior Engineer. (yosef@pivotengineers.com)
Joseph Klein is a Project Engineer. (klein@pivotengineers.com)
Michael Ahern is a Principal. (ahern@pivotengineers.com)
Randall Poston is a Senior Principal and the President of the American
Concrete Institute. (poston@pivotengineers.com)
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References
ACI Committee 318. (2019). Building Code Requirements for Structural Concrete (ACI 318-19). Farmington Hills, MI: American
Concrete Institute.
ACI Committee 440. (2008). Guide for Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures
(ACI 440.2R-08). Farmington Hills, MI: American Concrete Institute.
International Code Council. (2015). 2015 International Building Code. Country Club Hills, IL: International Code Council.
Raths, C. H. (1974). Embedded Structural Steel Connections. PCI Journal, 104-112.
34 STRUCTURE magazine