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IITK-BMTPC Earthquake Tip 20

How do Beam-Column Joints in RC Buildings resist Earthquakes? page 2
beam top bar in position while casting the column up
Temporary (a) Stage I : to the soffit of the beam. Moreover, the vertical
prop
Beam top bars are not distance beyond the 90º bend in beam bars is not very
placed, but horizontal
ties in the joint region effective in providing anchorage. On the other hand, if
are stacked up. column width is large, beam bars may not extend
below soffit of the beam (Figure 5b). Thus, it is
preferable to have columns with sufficient width. Such
an approach is used in many codes [e.g., ACI318, 2005].
In interior joints, the beam bars (both top and
bottom) need to go through the joint without any cut
in the joint region. Also, these bars must be placed
within the column bars and with no bends (Figure 6).

Beam bars bent in joint region overstress
the core concrete adjoining the bends

Column (a) Poor Practice

(b) Beam Beam b a rs are w i thin colum n
Beam bars are within column
Stage II :

Top bars of the beam Beam bars and also straigh
bars and also straightt
are inserted in the Column
beam stirrups, and (b) Good Practice
beam reinforcement
cage is lowered into
the formwork Shear failure of RC
beam-column joint
(c) during the 1985
Stage III : Mexico City
Ties in the joint region are Photo from: The EERI Annotated Slide CD, 98-2, EERI, Oakland, CA, USA Earthquake,
when beam bars
raised to their final locations, are passed outside
tied with binding wire, and the column cross-
column ties are continued
section

Figure 4: Providing horizontal ties in the joints –
three-stage procedure is required. (c)

In exterior joints where beams terminate at Figure 6: Anchorage of beam bars in interior
columns (Figure 5), longitudinal beam bars need to be joints – diagrams (a) and (b) show cross-
anchored into the column to ensure proper gripping of sectional views in plan of joint region.
bar in joint. The length of anchorage for a bar of grade
Fe415 (characteristic tensile strength of 415MPa) is Related - Earthquake Tip
about 50 times its diameter. This length is measured Tip17: How do Earthquakes Affect Reinforced Concrete Buildings?
from the face of the column to the end of the bar Tip18: How do Beams in RC Buildings Resist Earthquakes?
anchored in the column. In columns of small widths Tip19: How do Columns in RC Buildings Resist Earthquakes?
and when beam bars are of large diameter (Figure 5a), Reading Material
a portion of beam top bar is embedded in the column ACI 318, (2005), “Building Code Requirements for Structural Concrete
that is cast up to the soffit of the beam, and a part of it and Commentary,” American Concrete Institute, USA
overhangs. It is difficult to hold such an overhanging IS 13920, (1993), “Indian Standard Code of Practice for Ductile Detailing
of Reinforced Concrete Structures Subjected to Seismic Forces,” Bureau
of Indian Standards, New Delhi
Narrow Column Wide Column SP 123, (1991), “Design of Beam-Column Joints for Seismic Resistance,”
Approximately 50 times bar diameter L-shaped ACI 318-2005 Authored by:
Special Publication, American Concrete Institute, USA

bar ends
Practice
C.V.R.Murty
Indian Institute of Technology Kanpur
Kanpur, India

Building Materials and Technology Promotion
Portion of top beam Portion of column Sponsored by:
Council, New Delhi, India
bar below soffit of the
beam already cast
(a) Poor (b) Good This release is a property of IIT Kanpur and BMTPC New

Figure 5: Anchorage of beam bars in exterior Delhi. It may be reproduced without changing its contents
joints – diagrams show elevation of joint region. and with due acknowledgement. Suggestions/comments
may be sent to: nicee@iitk.ac.in Visit www.nicee.org or
www.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips.
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