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ADDING SUPPLEMENTAL SHEAR
CAPACITY IN BEAMS USING DOWELS
Presented by Steven M. Talafous, P.E.
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40 Concrete Beams With A Problem:
• Beam stirrup spacings exceeded the allowable per
the building code
• 100 psf live load
• Moderate amount of flexural reinforcement
• 70 beams: typical concrete beam 20” wide x 20”
deep, 26’ span, 7” thick concrete slab
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A heavily used public parking structure in the Minneapolis-St. Paul
metropolitan area:
• Structure had been in use for over 10 years
• No indications of structural failure
• Decision to proceed with repairs to bring
the structure back into compliance with the
building code
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Exterior view
of building with
deficient beams
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Typical beam in public lobby area
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Pedestrian density vs. floor loading100 psf example
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How to Repair?
• Structure was and is heavily used
• Owner wanted to minimize disruption to the users
• Load testing was not an option, due to risk of
shear failure
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Congested areas below beams
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Repair method:
• Dowels added to provide shear reinforcement
• Proprietary adhesive system selected
• Pre-drill dowel hole using a proprietary percussion bit with a vacuum system
• All work done late at night, to minimize impact on the public
• Unique system - the building codes had not fully adapted this at the time of the repair
• This repair approach was independently confirmed by others
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Development of Dowels:
• ACI 318-08, 12.13.1 Commentary: Single leg stirrups of deformed bars discontinued in 1989 ACI 318 because difficult to hold in place during concrete placement and lack of a hook may make the stirrup ineffective as it crosses shear cracks near the end of the stirrup.
• ACI 318-08, 12.13.4: Longitudinal bars bent to act as shear reinforcement…if extended into a region of compression, shall be anchored beyond mid-depth d/2 as specified for development length in 12.2 for that part of fyt required to satisfy Eq. (11-17).
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Development of Dowels:
• Therefore, look to code prior to 1989…
ACI 318-83 (1986), 12.13.2.2: Embedment d/2
above or below mid-depth on the compression
side of the ember for full development length
ld but not less than 24db ; or for deformed bars
or deformed wire, 12 in.
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Installing dowel in angled configuration
from underside of beam
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Epoxy fill of dowel hole
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Vacuum dowel drill system minimized
protection needed
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Dowels installed into underside of beam
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Repair method:
• 3 bars sizes existing reinforcement - #4, #5, & #6 bars
• 19 dowel configurations used for different beam and reinforcing arrangements
• 4 different supplemental dowel shear insertion types
• Used where dowels could not be placed:
• Shear capacity
• Spacing limitations
• This method used at 10 of 633 supplemental shear reinforcement locations
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Hooked dowels installed in roof
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Placing polymer repair mortar
prior to roof patching
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Construction:
• 18-week schedule, completed in 16 weeks
• $645,000 budget – actual cost $600,000
• 633 supplemental shear reinforcement locations
• 462 installed on topside
• 161 installed from bottom side
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Repair of finishes – touchup
of dowel location
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The outcome:• Adhesive dowels into the beams minimized
• Options so dowels could be installed in
congested areas
• With several options, contractor was able
avoid existing pipes/ducts/conduits
• With this repair, contractor able to remove
minimal amount of finishes
• s-built beams closely matched the design
drawings – this does not always happen!