The Suggestion offered by Robert, are seen as most appropriate, which can take care of supplementing the shortfall in either Moment or Shear Capacities.
However the option in place of welding could be use of Shear Connector Bolts Stitching to Webs and Flared additional steel Plates, with the stiffeners welded to the Web plates, which could in some way, eliminate the need for control of welding temperature at interface of Steel plates and Concrete.
Yet another option could be " Carbon Fibre Wrap " or Carbon Plates attachments, though this will need very very high care in effective bonding to concrete and Fire Protection, since any fire during lifetime of Building, can otherwise destruct this Wrapping, almost instantly, and thereby negating the very input for strengthening, which may lead to a catastrophe
Are the additional loads as envisaged, for change of " User" too significant ? If from Residential to Commercial, the reduction in Partition Wall Loads of Floors, ( designed for Residential Floor ), and NOT needed for a Commercial Offices Use, in such a Scenario, may Not be a major issue.
In my experience, these Transfer Girders' Design, would be more " Shear-Critical, rather than Bending Moments
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Satish Dhupelia A.M.ASCE
Consulting Structural Engineer
Mumbai MH
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Original Message:
Sent: 07-20-2017 08:26
From: Robert Bullard
Subject: Strengthening of transfer girders of a 22 story structure
Box the beams in steel with flange extensions to get weld heat (use low heat skip weld pattern) away from concrete; apply a combined doweled and high-head adhesive bond at all faces. A real time thermal measurement and dispersion protocol must be used during welding. Undertake a similar approach at the columns. As you iterate a design, you will find the steel cladding will quickly pick up the lion's share of stresses, perhaps to the point where any allocation to the original conventional concrete beam/column is superfluous, especially if the building is in a high seismic location.
Fire rating is an issue, which, if you can not solve with cladding, and you do not already have a modifiable sprinkler system, you may have to retrofit a localized water or chemical fire suppressant system.
This type of design challenge is the rubber meeting the road of structural engineering.
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Robert Bullard P.E., M.ASCE
President
Ahimsa Technic Inc
Ponce Inlet FL
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Original Message:
Sent: 07-19-2017 02:52
From: Shashank Pendharkar
Subject: Strengthening of transfer girders of a 22 story structure
Due to change in market conditions client requested change in use of floors. Therefore, we are required to strengthen transfer girders which support floating columns of 17 residential floors.
At present all 17 floors have been constructed which represents ~80% total load. Post tension contractors are not available locally.
Transfer girders are located in Parking area where it is possible to strengthen from top so that parking are not affected. We propose to increase depth of girders to support remaining 20% load by adding reinforced concrete cast in situ at top and to achieve composite action through links which will be anchored to the existing top rebars of girders.
Due to minimum clearance required under the transfer girders it is not possible to strengthen from below.
1. Has anyone similar experience of strengthening works by adding concrete at top?
2. Which material other than concrete was used for strengthening?
3. What measures should be adopted to ensure composite action with existing concrete?
4. Indication of cost/m or /sq.m for strengthening works?
Regards
Pendharkar