4.2 - VSL

7. Preliminary design
In the design of a structure, both the
structural design requirements and the type
of use should be taken into account. The
following points need to be carefully clarified
before a design is carried out:
- Type of structure: car park, warehouse,
commercial building, residential building,
industrial building, school, etc.
- Shape in plan, dimensions of spans,
column dimensions; the possiblility of
strengthening the column heads of a flat
slab by drop panels
- Use: live load (type: permanent loads,
moving loads, dynamic loads), sensitivity
to deflection (e.g. slabs with rigid structures
supported on them), appearance
(cracks), vibrations, fire resistance class,
corrosive environment, installations
(openings in slabs).
For the example of a square internal span of
a flat slab (Fig. 56) a rapid preliminary design
will be made possible for the design engineer
with the assistance of two diagrams, in which
guidance values for the slab thickness and
the size of the prestress are stated.
Figure 56: Internal span of a fla slab
The design charts (Figs. 57 and 58) are
based upon the following conditions:
1. A factor of safety of y = 1.8 is to be
maintained under service load.
2. Under self-weight and initial prestress the
tensile stress 6c;t for a concrete for which
f28 = 30 N/mm2 c
shall not exceed 1.0
N/mm2.
3. The ultimate moment shall be capable of
being resisted by the specified minimum
ordinary reinforcement or, in the case of
large live loads, by increased ordinary
reinforcement, together with the
corresponding post-tensioning steel.
The post-tensioning steel (tendons in the
span and over the columns) and the ordinary
reinforcement are assumed as uniformly
distributed across the entire span. The
tendons are to be arranged according to
Chapter 5.1. and the ordinary reinforcement
according to Fig. 35.
From conditon 1, the necessary values are
obtained for the prestress and ordinary
reinforcement as a function of the slab
thickness and span. Conditon 2 limits the
Figure 57: Recommended ratio of span to slab thickness as a function of service load to
self-weight (internal span of a flat slab)
Figure 58: Ratio of transverse component a from prestress to self-weight g as a function of
service
maximum admissible prestress. In flat slabs,
the lower face in the column region is usually
the determining feature. In special cases,
ordinary reinforcement can be placed there.
The concrete tensile stress oct (condition 2)
should then be limited to σct 2.0 N/mm 2 .
With condition 3, a guidance value is
obtained for economic slab thickness
(Fig.57). It is recommended that the ratio I/h
shall be chosen not greater than 40. In
buildings the slab thickness should normally
not be less than 160 mm.
Fig. 57 and 58 can be used correspondingly
for edge and corner spans.
Procedure in the preliminary design of a flat
slab:
Given: span I, column dimensions, live load
q
1. Estimation of the ratio I/h → self-weight g.
2. With ratio of service load (g+q) to
selfweight g and span I, determine slab
thickness h from Fig. 57; if necessary
correct g.
3. With I, h and (g+q)/g; determine
transverse component from Fig. 58 and
from this prestress; estimate approximate
quantity of ordinary reinforcement.
4. Check for punching; if necessary flare out
column head or choose higher concrete
quality or increase h.
The practical execution of a preliminary
design will be found in the calculation
example (Chapter 8.2.).
19