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ADVANCED SYSTEMS FOR RATIONAL SLAB REINFORCEMENT

CASPER ÅLANDER

M. Sc. (Civ. Eng.) Development manager

Fundia Reinforcing Abstract This paper deals with rational and fast ways to reinforce concrete slabs. Flat slabs are dealt with especially concerning the punching problem and a special solution for preventing punching, the UFO. The state of the art concerning the flexural reinforcement of slabs is the Bamtec carpet reinforcing technique, which is described and illustrated by photos and figures. Keywords Slab, Reinforcement, Punching, Shearhead, UFO, Slim Columns, Carpet Reinforcement, Bamtec. Biographical notes Development manager at the Nordic steel company Fundia since 1987 CAE Consultant at Pöyry & Nokia Advanced Engineering Oy 1985-1987 Assistant to the Scientific Attache at the Finnish Embassy in Oslo 1984-1985 Designer Juva Engineering Oy 1979-1984 Chairman of the Finnish commission for reinforcing steels Member of the Finnish committee for development of design codes for concrete Member of the Finnish mirror group for EC2 Member of the Finnish Concrete Association Member of the Finnish Concrete Floor Association Member of fib and fib TG4.5 Chairman of the steering committee for the research project; ”Restricting Corrosion Risk in Reinforced Concrete Structures under Outdoor Conditions” at Helsinki University of Technology 1998-2002. (University reports: www.hut.fi/Yksikot/Talo/julk.htm#julk111, www.hut.fi/Yksikot/Talo/julk.htm#julk121) Graduated 1983, Helsinki University of Technology, Building Technology and Materials, Masters thesis; “Ductility and Strength of Concrete Slabs Reinforced with Coldworked Deformed Rebars”

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1. INTRODUCTION The flat slab is known to be one of the most competitive frame solutions for multi story buildings. Basically the competitiveness derives from the simplicity of the structure. In addition to the bracing system, necessary in all tall buildings, the frame consists only of continuous slabs and columns between the slabs. A number of advantages can be listed:

− Low structural height − Low cost due to lack of beams and two way function of the slab − Good sound insulation and low structure-borne noise − Free geometry, more inspiring architecture − Flexibility regarding changes in layout − Free routes for channels, pipes and wires − Easy and rapid construction especially when advanced reinforcing technique is applied

Fig. 1. Flat slab with UFO punching preventer and Bamtec carpet reinforcement. Very large spans are not economical or even possible with slim structures like massive flat slabs. Therefore it is better to focus on techniques that help to minimize the inconvenience of the columns. From a user point of view this means methods to decrease the column thickness. The new punching preventer, the so-called UFO, makes it possible to use super slender columns without complicating the frame e.g. by bulky and laborious column heads. Super slender columns are much more acceptable, simply because of their slim appearance. Thus the demand for high span becomes less relevant. The state of the art concerning slab reinforcement is the Bamtec system, which allows optimal reinforcement in combination with unique installation speed on site. Bamtec carpets usually contain up to 1,5 tonnes of optimised reinforcement. The installation time per carpet is 5…8 minutes. Consequently the time needed to install the reinforcement on site has proven to be only 20…25 % compared to more traditional reinforcing techniques. The economical benefits of this can be huge compared to the costs of the reinforcement.

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2. The punching problem

2.1 General

In ultimate limit state the situation just before punching failure is illustrated in fig. 2.

Fig 2. The critical spot in a flat slab frame concerning load bearing capacity. The concentration of bending moments and shear force results in a stress concentration in the lower compressed part of the concrete around the edge of the column (see fig. 2). The contribution of the bending moments to the stress resultant is essential. In the critical area the concrete compression even exceeds the linear compressive strength of the concrete. The potential for increasing the load bearing capacity by ordinary punching shear reinforcement is limited, due to the fact that such reinforcement does not decrease the horizontal stress component. For this reason, most concrete codes limit the possibility of increasing the punching capacity by punching shear reinforcements to usually 1,6 times the capacity without punching shear reinforcement. The stress situation in the critical area can be characterized as two-dimensional. This explains why the linear strength of the concrete can be exceeded in the critical area. This increased strength situation has been taken into account for in most codes by using an increased strength in the formula for punching capacity compared to linear shear. In the Eurocode 2 proposal (prEN 1992-1-1: December 2003) the compensation is however made by increasing the control perimeter instead, to a distance 2*d from the edge of the support, where (d) is the effective depth of the slab. Unfortunately the Eurocode 2 approach can be sufficiently accurate only for a small part of possible relations between the column thickness and the effective depth of the slab. Comparing punching tests, with a large variety in the ratio between column thickness (c) and slab depth, it seems like the best correlation with test results is achieved with the control perimeter is at distance 0,75*d from the edge of the support, see Fig 3. This location would also not overestimate the resistance to eccentric loading.

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Tuning of control perimeter

0,65

0,75

0,85

0,95

1,05

1,15

1,25

1,00 2,00 3,00 4,00 5,00 6,00

c/d

Pu/V

11,520,750,5Linear (1)Linear (1,5)Linear (2)Linear (0,75)Linear (0,5)

Fig 3. Based on test with large variation in the relation between column diameter (c) and effective slab depth (d) the optimal location of the control perimeter seems to be at 0,75 d from the edge of the loaded area. The punching failure surface was studied from detached test pieces. In practice it is not cone shaped as most simplifications describe it. A proposal for generalisation of the section of the failure surface is presented on the right side in fig. 4. On the left side some alternative locations considered for the control perimeter are indicated.

Fig. 4. Generalisation of the punching failure surface.

2.2 The function of the UFO punching preventer The UFO punching preventer is a load bearing product, a shearhead, which is made of steel plate material (fig. 5) . Its location of use is inside the slab centrically on top of the column. The UFO increases the punching capacity beyond the level that can be reached by punching shear reinforcement. Thus slimmer structures can be utilized.

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Fig. 5. The UFO punching preventer. The bottom flange and the lower part of the product functions like a support for the slab in the outer area. In the inner area the UFO collects the reactions from the slab and transfers them by membrane action to the area on top of the column. Thus the UFO has the same function regarding punching as a mushroom shaped column head. It is a support for the slab, which is larger than the column.

Fig 6. The function and location of the UFO. The punching capacity of the slab in the outer area can be calculated in accordance with relevant concrete code. The punching capacity of the slab can be calculated in accordance with relevant concrete code, using the size of the UFO as the size of the support. The load bearing capacity of the UFO is determined by a special model, which is verified by testing. The product is available in two sizes and each size in alternative thickness of the plate material.

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Design strength of UFO (no eccentrisity, γm=1.15)

200

400

600

800

1000

1200

1400

1600

1800

200011

013

015

017

019

021

023

025

027

029

031

033

035

037

039

041

043

045

0

Column diameter (mm)

VRd

(kN

)

UFO900/8

UFO900/7

UFO900/6

UFO550/5

UFO550/4

UFO550/3

Fig. 7. The design load bearing capacity pf the UFO. The design load bearing capacity for each size of the product is shown in figure 7. It is depending only on the diameter of the column. Other column shapes are transformed to circular shape with the same section area before reading the load bearing capacity.

3. How to make slender columns The techniques to make columns slender are:

− Minimize bending moments (structural model with hinges at both ends) − High strength concrete − Moderate or high reinforcement ratio − Composite steel tube solution for extreme slenderness − Advanced calculation methods (second order theory, non linear material model,

accurate fire calculation)

Fig. 8. Principle of column reinforcement and example of section of a super slender composite column cast into a steel tube.

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4. The fastest way to reinforce a slab

4.1 The evolution of slab reinforcing In the history of slab reinforcing the following main development stages can be recognized: At the beginning of the history of reinforced concrete, about 100 years ago, smooth loose bars with end hooks were used. Material was expensive in comparison to labour costs and time saving was not yet a big issue.

Fig. 9. About 100 years ago smooth loose bars with end hooks were used. During the 4th decade of last century ribbed rebars came into use. Thus the behaviour of the structure was improved a great deal, especially regarding stiffness, crack behaviour and energy absorption. The need for end hooks was also effectively reduced. (Old fashioned designers and conservative concrete codes still apply end hooks also where they really are not needed). During the 5th decade of the last century welded fabric became popular. Above all they provided less laborious fixing on site. One problem with ordinary fabric however is the relatively small size, meaning a lot of lap joints. After this the development of slab reinforcement was focused on special mesh applications. One such application is the one-way special mesh with lengths tailored to suite the actual need in the building. With these products the reinforcement layer thickness in the lap zones could be reduced and also the amount of laps joints was reduced. Consequently the quality of the RC structure was improved.

Fig. 10. About 30 years ago special one-way welded fabric was invented.

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The state of the art of slab reinforcement is the carpet reinforcing technique. This is like a loose bar reinforcement packed into a roll. The advantage is extreme fixing speed. When advanced design and production systems are utilized the reinforcement can in addition be optimised and provided with necessary openings.

Fig. 11. Today the state of the art of slab reinforcing is the Bamtec carpet system. This carpet reinforcing system is now known under the registered trademark BAMTEC®. The Bamtec system was developed and a patent applied for in 1993 by Dipl.-Ing. (TU), Dipl.-Kfm. Wilhelm Häussler and Dipl.-Ing.(TU) Norbert Nieder.

4.2 The Bamtec carpet reinforcement

Fig. 12. The carpet reinforcement is generally speaking an optimised loose bar reinforcement packed in a special way for fast fixing on site.

The reinforcing steel in a Bamtec carpet is the ordinary weldable ribbed reinforcing steel certified for use in the market area in question, in the Nordic countries typically 500 or 550 grade Tempcore steel in sizes from 8 to 32 mm. The maximum length of the carpet roll is 15 m. In the rollout direction the carpet can be tens of meters long. Considering the rollout operation more than 1,5 t of steel in one carpet is not recommended. The carpet is held together by thin strips of steel to which the bars are welded in a special machine. The spacing of the steel strips is 1,55 m.

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Fig. 13. One carpet is typically hundreds of square meters in size and includes different amounts of reinforcement in different locations.

When dealing with a reinforcement element, that typically is hundreds of square meters in size, it is clear that the system must allow different amounts of steel in different locations of the carpet in order to reach an optimal reinforcement. With the Bamtec system this requirement is met, by integrating sophisticated design tools with the fully or semi automatic production line.

4.3 The design of Bamtec slab reinforcement There are several software alternatives available for the design of the Bamtec carpets. Common for them all is that they produce the input data needed to control the machine production.

Fig. 14. The Bamtec system comprises sophisticated software for design.

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At the most sophisticated level the forces in the slab are calculated with the Finite Element Method (FEM). This information is transformed into the required amounts of steel in different parts of the slab. Then a Bamtec module is used for computer aided design (CAD) of the carpets. Advanced Bamtec modules can read the output information from the calculation and generate the carpets more or less automatically. Finally at the highest level of calculation the actual reinforcement data is fed back into the calculation for control of crack widths and deflections. This last stage is perhaps not relevant in all market areas, but is certainly very important in the Nordic countries, where the serviceability limit state tends to dimension the slabs. A less sophisticated way to design Bamtec carpets is where the calculation software is not integrated with the Bamtec module. Then the designer has to take care that the correct amount of reinforcement is provided in the different parts of the carpet. Graphic output from the FEM calculation (in dxf-format) can be used as background information to make this job easier. Using this method the level of optimisation may not be that high, but a skilled designer can design rational carpets. Also with the less sophisticated design tool both the production data for the machine and the rollout plan for the site are generated automatically.

Fig. 15. Example of rollout plan of the bottom reinforcement in ETA-direction in an intermediate floor in a block of flats.

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4.4 The carpet production

Once the design is done with suitable software, which produces the input data for the machine, the production is automatic. Production lines without an integrated straightening unit and for large bar sizes not available in coil the raw material is pre cut bars, which are fed into the machine one by one with the help of a special device. This type of production is recognised as semi automatic. A fully automatic Bamtec machine is a combination of a straightening unit and a carpet welding unit. These units communicate, so that the carpet welding machine automatically is fed with bars with correct diameter and length. With this type of production it is possible to produce carpets containing both thin, straightened bars and thick pre-cut bars.

Fig. 16. A fully automatic Bamtec machine loaded with coil material in three sizes. On the right side the feeding of pre-cut bars.

The machine is rolling the carpet around reinforcing steel spirals or rings. These are attached to the first bars usually by tack welding.

4.5 The installation of carpets The very high installation speed is one of the major benefits with the Bamtec system. The installation of one carpet has proven to take 5 to 8 minutes, regardless of the amount of steel or the shape of the carpet. In large projects with heavy carpets this means e.g. that 3 men can install about 6000 kg/h corresponding to 2000 kg/man/h. This is about 10 times more than with traditional reinforcing methods. In practice when comparing the overall installation speed in completed projects, the time savings have been around 80 % at least 75 % in comparison to traditional reinforcing methods.

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For the largest carpets three to four men are often needed. Light carpets can be rolled out by two men. Crane assistance has to be available. Another important benefit with the Bamtec system is the essentially improved ergonomics. In addition to this the rollout does not require special skills, like in the case with loose bar solutions and ordinary reinforcing drawings. The rollout plan (see fig 15) is clear and easy to read. The installation procedure consists basically of 4 stages:

1) The carpet is found and lifted from the site stock to its position on the slab. 2) The carpet is positioned with great precision on the spot with the first bar exactly on the

start line. The packing ties are removed. 3) The rollout itself, takes only some seconds. 4) The spirals or rings are removed by breaking the tack welds, usually by a moderate

kick. For stage 2) a special method applying rope stumps with hooks at one end has been developed for situations where lifting loops around the carpet roll are used, thus preventing the package from being opened while hanging.

Fig. 17. Rollout of large Bamtec carpets normally takes 5 to 8 minutes per carpet, which may weigh up to 1500 kg.

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4.6 What is Bamtec reinforcement best suitable for? A frequently asked question is what kind of projects the Bamtec system is suitable for. There may be no specific answer to this question, because the main benefit, as I see it, is the extremely fast installation. Fast installation means time saving and this potential for saving should theoretically be present in all projects. The question is whether the customer is ready to calculate and utilize the benefits of short fixing time or is he focused on low unit price for the reinforcement. Today the time schedules are often tight, because the expectations on the return on the building investment are high. Alternative frame systems, e.g. systems based on pre-cast concrete or steel elements have influenced the general expectations on time schedules in this direction. This means that cast on site concrete construction in general needs faster reinforcing techniques. Thanks to the Bamtec system the cast on site concrete frame can be competitive in speed with the fastest alternatives, at least when rational forming and concreting methods also are applied. The Bamtec system has been used in large and small projects, in complicated and easy designs. In all cases the customers that have chosen the Bamtec seem to be very pleased with the decision. From a production point of view, large carpets with mostly 12 and 14 mm bars and perhaps a few larger bars give the best productivity, with a fully automatic machine. With a semi automatic machine large bars (>16 mm) are most attractive. Carpets that are efficient to produce have the lowest unit price. This however does not mean, that the total saving is the best in projects with such carpets. It could be, that an optimised reinforcement with mostly 10 mm bars results in a very good overall saving although the unit price for such a carpet is higher.

Fig. 18. An interesting way of making flat slabs, with the Filigran type of casting form including most of the bottom reinforcement (and the supports for the top reinforcement), UFO punching preventer and Bamtec carpets for the top reinforcement. In any case the cost of the fixed reinforcement itself is not the main issue. The effect of time saving is. In a project where the time saving effect is fully utilized, it could mean savings at the bottom line, which are of the same order as the cost for the complete reinforcement.

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5 Conclusion This paper describes two rational and relatively new techniques available for efficient reinforcement of slabs. The punching UFO preventer is a load bearing steel product, a shear head for flat slabs, which makes slimmer structures possible than with ordinary shear reinforcing technique. The UFO is a seamless and simple low weight product, which is mass produced and available in standard sizes. When this product is used the punching is no longer dimensioning the thickness of the column and the slab. Thus the flat slab frame can be optimised on other premises and its competitiveness is thereby improved. Bamtec is a slab reinforcing system where the reinforcement is delivered to the site in large roll units, so-called reinforcement carpets, consisting of parallel bars held together by thin steel strips. The size of a Bamtec carpet is typically several hundred square meters. The rebars in the carpet are ordinary weldable reinforcing steel bars. The installation is a rolling out procedure, which gives a unique speed advantage in comparison to more traditional reinforcing methods. Compared e.g. to mesh reinforcement considerable material savings are also achieved due to less lap splices. The Bamtec system is also ergonomically a convenient way to reinforce slabs as the heavy work is done by the crane.