TWO WAY SLAB
TWO WAY SLAB
DIRECT DESIGN METHOD)
DIRECT DESIGN METHOD)
DIRECT DESIGN METHOD FOR TWO – WAY SLAB
In designing two-way slab method, the loads of the floor must be distributed in both directions such that the spans shall be equal or the difference between 2 adjacent panels shall not be greater than 4/3. The only difference between the design approaches of two-way slab in one-way slab is that the floor slab should be divided into a series of slab strips across both directions of the floor panel. The direct design method gives rules for the determination of the total static design moment and its distribution between negative and positive moment sections.
Fig. 1-1
In two-way design, as close as slab strips to the column support, it will carry a much higher moment than that of the mid-span. This two-way design helps you attain an exact amount of force distribution among the slab strip. After obtaining the moment in slab strips, the design of reinforcement will follow.
The slab thickness is the most important part of the dead load structure such that the weight of the slab can be determined already. The minimum slab thickness must be thick enough to provide the sufficient shear strength.
The following are the direct design met hod limitations according to NSCP 2010 code sec. 413.7: 1) There shall be a minimum of three continuous spans in each direction.
2) Panels shall be rectangular, with a ratio of shorter to longer span center-to-center supports within a panel not greater than 2.
3) Successive span lengths center-to-center supports in each direction shall not differ by more than one-third the longer span.
4) Offset of columns by a maximum of 10
%
of the span in direction of offset from either axis between center lines of successive columns shall be permitted.5) All loads shall be due to gravity only and uniformly distributed over an entire panel. Live load shall not exceed two times dead load.
6) For a panel with beams between supports on all sides, the relative stiffness of beams in two perpendicular directions
Eq. 1-1Shall not be less than 0.2 nor greater than 5.0, where
and
2
are calculated in accordance with equation (1-3).
=
Eq. 1-27) Moment redistribution as permitted by section 408.5 shall not be applied for slab systems designed by the direct design method.
8) Variations from the limitations of section 413.7.1 shall be permitted if demonstrated by analysis as long as requirements of section 413.6.1 are satisfied.
Fig.1-2
ACCORDING TO NATIONAL STRUCTURAL CODE OF THE PHILIPPINES
(NSCP) 2010 6
THEDITION,
409.6.3.2
For slabs without interior beams spanning between the supports and having a ratio of long to short span not greater than 2, the minimum thickness shall be in accordance with the provisions of table 1-1 and shall not be less than the following values:
1. Slabs without drop panels as defined in Sections 413.3.5……….125mm
2. Slabs with drop panels as defined in Sections 413.3.5……… …100mm
Table 1-1
409.6.3.3
For slabs with beams spanning between the supports on all sides, the minimum thickness shall be as follows:
1. For
fm equal to or less than 0.2, the provisions of section 409.6.3.2 shall apply.2. For
fm greater than 0.2 but not greater than 2.0, the thickness shall not be less than:ℎ =
+5(
−.2)
0.8
4
and not less than 125mm;
3.
For
fm greater than 2.0, the thickness shall not be less than:ℎ =
.+
+9
and not less than 90mm.
Eq.1-3
4. At discontinuous edges, an edge beam shall be provided with a stiffness ratio
fm not less than0.8; or the minimum thickness required by Eq. 1-3 or 1-4 shall be increased by at least 10% in the panel with a discontinuous edge.
413.3.5
When used to reduce the amount of negative moment reinforcement over a column or minimum required slab thickness, a drop panel shall:
1. Project below the slab at least one quarter of the adjacent slab thickness; and
2. Extend in each direction from the center line of support a distance not less than one-sixth the span length measured from center-to-center of supports in that direction.
413.7.3.3
In an end span, total factored static m oment Mo shall be distributed as follows:
413.7.4 Factored Moments in column strip
413.7.4.1
Column strips shall be proportioned to resist the following percentage of interior negative factored moments:
413.7.4.2
Column strips shall be proportioned to resist the following percentage of exterior negative factored moments:
Linear interpolation shall be made between values shown, where tis calculated in Eq. 413-5 and C is
calculated in Eq. 413-6
=
2
= ∑10.63
The constant C or T or L-sections shall be permitted to be evaluated be dividing the section into separate rectangular parts, as defined in sec. 413.3.4, and summing the values of C for each part.
413.7.4.3
Where supports consists of columns or walls extending for a distance equal to or greater than three-fourths the span length l2 used to compute Mo, negative moments shall be considered to be
uniformly distributed across l2.
413.7.4.4
Column strips shall be proportioned to resist the following percentage of positive factored moment:
Linear interpolations shall be made between values shown
413.7.4.5
For slabs with beams between supports, the slab portion of co lumn strips shall be proportioned to resist that portion of column strip moment not resisted by beams.
Eq. 1-5 Eq. 1-6
