Main Structural Material Performances for New High-Rise Buildings

Some background considerations, from literature and from direct experiences here reported (Namiki, 2005), states that the improved new generation of High Performance Concrete (HPC) may lead the concrete to be highly competitive in respect to different materials, if the material performance is optimized taking account the response of the structure under the following actions:

(a) the structural behavior under wind actions,

(b) the structural behavior under explosion and impact actions,

(c) the behavior under cyclic and/or alternate loadings,

(d) the shrinkage and creep behavior.

Recent investigations comparing the different performances of various building materials can be summarized in Table 1, where the signs +, 0, – indicate respectively good, neutral, bad performance. High Strength Concrete (HSC) shows positive performances respect to the entire spectrum of the chosen criteria (Chew Yit Lin, 2003; High-Rise Manual, 2003; Taranth, 1988).

Table 1. Comparison on different materials frame construction.

Criteria

Reinforced concrete, Normal-strength concrete

Reinforced concrete High-strength concrete

Steel

construction

Composite

construction

method

Construction costs

+

++

0

+ +

Weight of construction

0

+

+ +

+

Stiffness

++

++

0

+

Flexibility of plan

0

0

+ +

+

Behavior in fire

++

++

+

Construction time

+

+ +

+ +

Usable area

+

+ +

+

SCORE

5

9

7

9

Taking moreover a particular look, for example, to the specific criteria identified as “construction cost” and “construction time”, Tables 2 and 3 show significative data (High-Rise Manual, 2003). Table 2 indicates sharp favourite performances by using HSC for the structures, not only considering the cost, but even from the point of view of the usable area needed by the structure. Table 3 is self explaining if we consider that HPC, beside improved mechanical properties, have the further relevant feature: formworks can be taken away just 24 hours after the pouring the concrete, while dealing with NSC (Normal Strength Concrete) at least 4-5 days are required. In Table 3 it is easy to observe that, while 13 working days were required to complete the rough work for a standard floor during the construction of the Dresden Bank high-rise in 1974-1979 (obsolete concrete technology + NSC), the same task is currently achieved in mere 4 working days on the Galileo site (modern concrete technology + HPC) (High-Rise Manual, 2003; The Concrete Society, 1997; Fairweather, 2004). The immediate cost construction are primarily related to the time construction. Tough steel structure is traditionally reckoned to be much faster than the concrete structure, but because of significant advantages in concrete technology, this is no more true: in fact, with concrete showing features of self-levelling (SCC), the new pumps equipment assures concrete puring even at heights of 300 m, and also because of modern formwork systems (self-climbing formwork), rapid and safe progress in the rough work is guaranteed.

Research studies carried out, and still in progress, in the field of mix-design of HSC/HPC, and related technology in the construction sites, seem to show favourable results in achieving better per­formances, i. e. rapid hardening, absence of segregation, better durability (no alcaly-silica reaction), when limestone is used as filler, as it will show in the next section.

It is worth to recall, as mentioned before, due to even the consequences of recent past accidents, that the engineering design of a land-mark buildings can not ignore the possible blast loading effects. As it is well known, the physical action on a wall F, due to a plane shock wave, generates an overpressure Ps and a drag loading Pd, according to the scheme of Figures 3a and 3b.

When an explosion occurs inside a building, then it is the interior surface of the walls and ceiling which are first loaded by the pressure of the shock wave, reflecting therefore and increasing the pres­sure. The effects on the structure may be devastating, considering in addition that, as consequence of an explosion, even fire may occur. Figure 4 shows the accident occurred in spring 2002 at the Pirelli building, headquarter of the Regional Government in Milan. The explosion of the fuel tanks of the aircraft, among other consequences, caused a permanent deformation of the 26th r. c. floor with a

Table 2. Construction costs and usable area for different structural material.

Table 3. Construction time – HPC/HSC versus steel.

Property

Height

Completion

Rough Work per Standard Floor

Business Research Center, Warsaw, Poland

104 m

2000

5 working days

Taunustor Japan Center, Frankfurt, Germany

114m

1996

4 working days

World Port Center, Rotterdam, The Netherlands

125 m

2001

5 working days

Galileo,

Frankfurt. Germany

136 m

2003

4 working days

Drcsdncr Bank, Frankfurt. Germany

166 m

1979

13 working days

Trianon,

Frankfurt, Germany

186 m

1993

5,5 working days

Millennium Tower, Vienna, Austria

202 m

1999

3 working days

Park Tower, Chicago, USA

257 m

2000

3 working days

Trump World Tower, New York. USA

269 m

2001

5 working days

Pctronas Tower,

Kuala Lumpur, Malaysia

452 m

1998

5 working days

Fig. 3. Overpressure due to plane shock wave action.

Fig. 4. Draw of the airplane impact and the permanent floor deflection.

deflection of 22 cm, but not the collapse, despite the high temperature due to fire (Migliacci et al., 2005; Kappos, 2002).

The stiffness of the structure is very significant in considering the horizontal loads, i. e. earthquake and wind actions.

• Earthquake effects: stell constructions are highly suitable in areas subjected to earthquake as steel allows the structure to absorb part of the kinetic energy produced by the earthquake in the form of plastic deformation. Technically a similar ductile behaviour can be achieved by using for the structure HSC and reinforcement steel with high strength and ductility.

• Wind effects: reinforced concrete as the merit over steel frame construction in high rise build­ings to present a less sway wind. The structural behaviour minimizing the wind effect is at best achieved by concrete displaying a high values of Young modulus.

Fig. 5. Plastic model of “City Life” project in Milan. .

Fig. 6. Plastic model of “Garibaldi-Repubblica” project in Milan.

The points so far briefly recalled seem to highly recommend the choice of using reinforced concrete structure in the engineering design for a high-rise building, with particular attention to the best suitable mix design of HSC/HPC for each different action and structural component of the building.

For these reasons, a comprehensive study on best performing HPC mix design, with particular reference to Limestone Concrete, have been requested, with the purpose to draw Guidelines for use, in the oncoming starting up design phase of two important land mark interventions in the city of Milano. Figures 5 and 6 show the plastic model of the two projects, respectively the so-called “City Life” in the former trade fair area, and the so-called “Garibaldi-Repubblica” area, both in the central part of the town.

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