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[eondes at sify.com]

Dear All,

This is going to be my 3rd comment.

Part 3:

1.    I feel a separate clause as follows be introduced

Selection of a Strengthening Solution

Utilizing the investigation data and criteria for strengthening, the
designers must perform analysis to determine why the building needs
strengthening, develop alternative schemes to strengthen. These alternatice
schemes must be eveluated and the most appropriate solution selected as
explained in the following steps:

First, engineer/designer must thoroughly understand why the building needs
strengthening?. The force resistant paths in the building must be determined
to explain why certain members may sustain damage while other members would
essentially be undamaged.

It must be determined if the building needs strengthening due to
a.    Discountinuities in strength or stiffness
b.    Torsional moments within structure
c.    Hammering with adjacent structures
d.    Improper connection details
e.    Effect of structural elements such as infill walls and appendages
Also it must be determined if members would fail due to
a.    shear,
b.    compression,
c.    tension,
d.    flexure,
e.    bar anchorage etc.

Second, engineer/designer must calculate and perform analysis in order to
evaluate the existing strength and stiffness of the building. If the
analysed and evaluated building conforms to the design criteria, then
building strengthening need not be suggested. If the building strength is
less than the requirement of criteria, strengthening will be required and
suggested.

Third, based on the results of the analysis above, alternative solutions for
strengthening can be determined for evaluating their feasibility.
Imagination and ingenuity should be exercised, as best solution is seldom
the first one conceived. Calculations in sufficient detail be performed to
provide  a reasonable basis for the comparision of proposed schemes.

The best solution must be evaluated on the basis of
a.    Compatibility of the solution with the functional requirements of
the building,
b.    Feasibility of construction, including availability of material,
construction equipment and personnel with specialized skills & training and
ability to implement
the solution
c.    Economical considerations
d.    Aesthetics

Forth, after selecting a structural solution for the strengthening, a
complete analysis must be performed for design to identify weak links which
will fail or exceed their yield strength first. The following weak links
already identified in past earthquakes must be given due attention:
a.    Short / Stiff Column in comparision to other columns at the same
level, possibly due to partial height infilled walls
b.    Columns beneath discountinuous shear walls
c.    Abrupt changes in stiffness due to change in layout or structural
system
d.    Concrete frame joints at infilled walls

e)   Stories which are more flexible than adjacent stories, possibly due
to less infilled walls

f)    Excessive building torsion due to inappropriate distribution of
stiffness

g)   Joint regions in concrete frames

h)   Flat slab to column connection

i)    Pre-cast elements with weak connections.



With a thorough understanding of weak links in the building, the designer
can design strengthening measures, which will improve the response of
building in the future earthquakes. These measures should also consider
avoiding irregularities in plan, abrupt changes in stiffness between floors
and elements subjected to shear or brittle failure. The effect of
strengthening elements added to the building must be carefully evaluated to
insure that they will not cause increased damage in a future earthquake. If
strengthening is proposed only in single story, the consequences of the
increased strength and stiffness in that single story must be carefully
analyzed and evaluated after considering structure in totality.



Fifth, final design procedure includes a completion of detailed calculations
of the strengthening solution and preparation of drawings, specifications
and instructions. The designer should keep in mind that repair techniques
may be unfamiliar to most of construction workers and hence detailed
procedures instructions must be provided. The document must show all work
that is required to be performed without leaving interpretations to the
contractor and the workers. Always need is justified for an independent
review and inspection, but designer must be experienced & always prepared to
modify the details arising out of unexpected quarters or contingencies


With warm regards.

Arvind Jaiswal,

Chief Consulting Engineer,
EON Designers,
35-B, Vasavi Colony, Behind Vikrampuri, Secunderabad- 500 015
e-mail: eondes@sify.com Ph. 040-27847847 Mobile 092463 40202

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