Concrete Design Ali N. Attiyah, Ph.D.
Fourth Year 2015-2016
1
Lecture five
Direct Design Method
The third step of DDM is to calculate column strip moment from each of the positive and
negative moments according to ACI 8.10.5.
Concrete Design Ali N. Attiyah, Ph.D.
Fourth Year 2015-2016
2
The column strip negative moment at exterior support is dependent on the parameter β
t
,
which is defined as follows:
For example, if an edge beam is considers the torsional constant C is found from Eq.
8.10.5.2b, where x and y is found from subdivision of the shape into rectangles in such a
way as to maximize C.
The parameter β
t
is taken as zero for unrestrained edges and 2.5 for fully restrained edges.
Concrete Design Ali N. Attiyah, Ph.D.
Fourth Year 2015-2016
3
The fourth step of DDM is to calculate the beam moment (if any), which is part of the
column strip moment according to ACI 8.10.5.7.
The fifth step of DDM is to calculate the middle strip moment according to ACI 8.10.6.
It is seen from the figure that the
middle strip moment is found
from two adjacent frames. The
moment in middle strip per
meter is calculated as follows:
M
mid.
/meter=
(M
frame A
+ M
frame B
)/Middle Strip width
Concrete Design Ali N. Attiyah, Ph.D.
Fourth Year 2015-2016
4
Example:
The sixth step of DDM is to calculate the column moment according to ACI 8.10.7.