Head Offices of Social Security for French Citizen Abroad / BVAU

© Clément Guillaume

· Architects: BVAU

· Location: 77 Rue des Meuniers, 77950 Rubelles, France

· Area: 3900.0 sqm

· Project Year: 2014

· Photographs: Clément Guillaume

· Project Management: BVAU-Bartolo Villemard Architecture Urbanisme

· Engineering firms: 3A Architectes Associés, TN + Paysagistes, RFR, RFR element, Avel acous- tique, BETOM (TCE)

· Client: Caisse des Français de l’Étranger

· Project cost: 9,3 M € before tax

· Environmental profile: BBC certification, Global performance is < 50kWh/m2/year.

© Clément Guillaume

From the architect. The project settles in an archetypal suburban and contemporary environment of our urban areas borders, so vague and unclear. Yet this place is still inhabited, people live and work there. We do are in town, and in this respect, we take a keen interest in this area.

© Clément Guillaume

Components of the projects are dealt with equal attention: an offices building and an outdoor car park, forming a facility with the existing building. We suggest a structure with an abstract, solid and perceptible evenness. The unbroken car park’s ground is made of raw concrete. In addition, we have planted 100 trees in this area. The result is a surprising and minimalist landscape.

© Clément Guillaume

The specific properties of the inner spaces are the basis of the connection between the place and its architecture. The building is adorned in its centre by an atrium.

Plan 00

We bring together two opposite types of spaces as well as two temporalities within this project. On the one hand, offices are an integral part of the everyday life. And on the other hand, specific areas with specific purposes create a continuum between the atrium and the townscape: common spaces that are both accessible and visible.

© Clément Guillaume

The building defines a logic of use and presence, it is welcoming and pleasant from the outside since it shows itself. Indoors, the available views from the common spaces offer a new perception of the urban environment, glorified by people’s eye. Our architecture offers a contextualised experience.

Axonometric

Headquarters of the CFE is a bioclimatic structure. It has a custom-made façade with a unique design. The lightened and prefabricated façade is made of an openable double skin glass covering wood for the inside front, and aluminium for the outside front. The façade is connected through sensors to an automated open/close system and awnings. The façade combined with the atrium allows natural ventilation. Heating and cooling are provided by heating and low-temperature floors, powered by geothermal energy.

Приложение 4

Piles and Foundation

1. It is not necessary to design nominal reinforcement to piles. Is it true?

In BS8110 and BS5400 Pt.4, they require the provision of nominal reinforcement for columns. However, for pile design the requirement of nominal reinforcement may not be necessary. Firstly, as piles are located underground, the occurrence of unexpected loads to piles is seldom. Secondly, shear failure of piles is considered not critical to the structure due to severe collision. Moreover, the failure of piles by buckling due to fire is unlikely because fire is rarely ignited underground. However, the suggestion of provision of nominal reinforcement to cater for seismic effect may be justified. Reference is made to J P Tyson (1995).

2. How do rock sockets take up loads?

The load transfer mechanism is summarized as follows:

When a socketed foundation is loaded, the resistance is provided by both rock socket wall and the socket base and the load distribution is a function of relative stiffness of foundation concrete and rock mass, socket geometry, socket roughness and strength. At small displacements the rock-socket system behaves in an elastic manner and the load distribution between socket wall and socket end can be obtained from elastic analysis. At displacements beyond 10-15mm, relative displacement occurs between rock and foundation and the socket bond begins to fail. This results in reduction of loads in rock-socket interface and more loads are transferred to the socket end. At further displacements, the interface strength drops to a residual value with total rupture of bond and more loads are then distributed to the socket end.

3. In designing mini-piles, should the strength of grout be neglected during assessment

of loading carrying capacity?

In designing min-piles, there are two approaches available:

(i) In the first approach, the axial resistance provided by the grout is neglected and steel bars take up the design loads only. This approach is a conservative one which leads to the use of high strength bars e.g. Dywidag bar. One should note that bending moment is not designed to be taken up by min-piles because of its slender geometry.

(ii) In the second approach, it involves loads to be taken up by both grout and steel bars together. In this way, strain compatibility requirement of grout and steel has to be satisfied.

4. What are the considerations in determining whether casings should be left in for mini-piles?

Contrary to most of pile design, the design of min-piles are controlled by internal capacity instead of external carrying capacity due to their small cross-sectional area.

There are mainly two reasons to account for designing mini-piles as friction piles:

(i) Due to its high slenderness ratio, a pile of 200mm diameter with 5m long has a shaft

area of 100 times greater than cross-sectional area. Therefore, the shaft friction mobilized should be greater than end resistance.

(ii) Settlements of 10%-20% of pile diameter are necessary to mobilize full end bearing

capacity, compared with 0.5%-1% of pile diameter to develop maximum shaft resistance.

Left-in casings for mini-piles have the following advantages:

(i) Improve resistance to corrosion of main bars;

(ii) Provide additional restraint against lateral buckling;

(iii) Improve the grout quality by preventing intrusion of groundwater during concreting;

(iv) Prevent occurrence of necking during lifting up of casings during concreting.

5. What is the purpose of post-grouting for mini-piles?

Post-grouting is normally carried out some time when grout of the initial grouting work has set (e.g. within 24 hours of initial grouting). It helps to increase the bearing capacity of mini-piles by enhancing larger effective pile diameter. Moreover, it improves the behaviour of soils adjacent to grouted piles and minimizes the effect of disturbance caused during construction. In essence, post-grouting helps to improve the bond between soils and grout, thereby enhancing better skin friction between them.

During the process of post-grouting, a tube with a hole at its bottom is lowered into the pile and grout is injected. The mechanism of post-grouting is as follows: the pressurized grout is initially confined by the hardened grout and can hardly get away. Then, it ruptures the grout cover and makes its way to the surrounding soils and into soft regions to develop an interlock with harder soil zones. In order to enhance the pressurized grout to rupture the initial grout depth, a maximum time limit is normally imposed between the time of initial grouting and time of post-grouting to avoid the development of high strength of initial grout. Consequently, the effect of soil disturbance by installation of casings and subsequent lifting up of casings would be lessened significantly.

6. In designing the lateral resistance of piles, should engineers only use the earth pressure against pile caps only?

In some design lateral loads are assumed to be resisted by earth pressure exerted against the side of pile caps only. However, it is demonstrated that the soil resistance of pile lengths do contribute a substantial part of lateral resistance. Therefore, in designing lateral resistance of piles, earth pressure exerted on piles should also be taken into consideration.

In analysis of lateral resistance provided by soils, a series of soil springs are adopted with modulus of reaction kept constant or varying with depth. The normal practice of using a constant modulus of reaction for soils is incorrect because it overestimates the maximum reaction force and underestimates the maximum bending moment. To obtain the profile of modulus of subgrade reaction, pressure meter tests shall be conducted in boreholes in site investigation. Reference is made to Bryan Leach (1980).

7. In some codes, they limit the ratio of weight of hammer to weight of pile for piledriving. What is the reason behind this?

When a hammer with initial motion collides with a stationary pile, the transfer of energy is most efficient when the two masses are comparable. That is the reason why some codes limit the ratio of weight of hammer to the weight of pile to be more than 0.5. If the weight of hammer is too low, most of energy during hammer driving is distributed to the hammer and this causes tension induced in hammer and results in inefficient transfer of energy.

8. Should engineers rely solely on Hiley’s formula in the design of H-piles?

About 90% of H-piles adopt Hiley’s formula for design. However, this formula is only applicable to pile lengths less than 30m and is suitable for course-grained materials (not suitable to fine-grained soils) as suggested by GEO (1996). In Hiley’s formula, by observing the penetration of piles after the hammer impact, the pile capacity could be readily obtained from the response of the impacting force. Therefore, the individual pile capacity could be obtained by this dynamic method.

However, in normal foundation, groups of H-piles are present and the soil foundation may not be able to support these H-piles simultaneously even though individual piles are proven to have sufficient capacity by using dynamic method. In this case, static method should be employed to ascertain if the soil foundation could support these H-piles.

9. What is the function of drilling fluid in rotary drilling in site investigation?

Drilling fluid in rotary serves two main purposes:

(i) Facilitate the rotation of drilling tube during rotary drilling

(ii) Act as a cooling agent to cool down heat generated during drilling operation.

Traditionally, water is normally employed as drilling fluid. However, it suffers from the

following drawbacks:

(i) It affects the stability of nearby ground with the introduction of water into the

borehole (borehole for soil; drillhole for rock);

(ii) It affects the quality of sample by changing the water content of soil samples collected from the borehole/drillhole. Substitutes are available in market to replace water as drilling fluid (e.g. white foam).

10. What are the differences in function between rock anchors and rock sockets?

Rock anchors are used primarily for resisting uplift forces. On the contrary, rock sockets serve three main purposes:

(i) Rock socket friction and end bearing to resist vertical load;

(ii) Passive resistance of rock sockets contribute to resistance of lateral load;

(iii) Socket shaft friction is also used for resisting uplifting forces. But only 70% of this capacity should be used because of the effect of negative Poisson ratio.

Note: Rock anchors, which may consist of a high tensile bar or a stranded cable, are provided for tension piles when there are insufficient soil covers to develop the required uplifting resistance.

11. What are the functions of cap block, drive cap and pile cushion in driven piles?

Cap block is installed between the hammer end and the drive cap to control the hammer blow in order to protect both the hammer and the pile from damage. When the hammer hits the cap block, it compresses elastically and reduces the peak forces, thereby lengthening the time of hammer blow. Moreover, it should be capable of transmitting the hammer energy effectively to the piles.

Drive cap is inserted at hammer tip to enhance uniform distribution of hammer energy to the pile. Pile cushion is positioned between the drive cap and the pile top. It intends to protect the pile from driving stress induced during hammer blows. Moreover, it also serves to provide a uniform driving load on top of the pile.

12.What is the significance of driving sequence of driven piles?

For basement construction, if piles are driven from the centre to the perimeter, there is a tendency of soils to move outwards. Such lateral movement of soil may cause damage to nearby structures and utilities.

However, if piles are driven from the outside perimeter inwards, there are little soil lateral movements. This results in a well-compacted centre with an excess pore water pressure built up to resist the loading of piles. Consequently, shorter pile lengths than the original designed ones may result. However, some time after the pile driving operation, the excess pore water pressure is dissipated and the shorter driven piles may not be able to take up the original design loads. In this situation redriving is required to drive the piles to deeper depths after dissipation of excess pore water pressure.

13. What is the function of followers in driven H-piles?

A follower is an extension between the pile head and the hammer that transfers the blow to the pile in which the pile head cannot be reached by the hammer or is under water .For construction of driven piles, the piling frame and hammer are normally erected on existing ground level but not at the base of pile caps

However, H-piles are designed to be terminated near the base of pile caps. If piles are driven at ground level, a certain length of H-piles is wasted and cut when constructing pile caps. In this connection, pile followers are used so as to save the wasted section of H-piles because followers can be removed during subsequent construction of pile caps.

14. What are the advantages of using top-down approach in basement construction?

The advantages of top-down approach are listed below:

(i) The structures above ground can be carried out simultaneously with the structures below ground. This greatly reduces the time for construction.

(ii) By using this approach, settlement can be reduced.

(iii) Since the permanent columns and slabs can be utilized to support loadings during construction, it saves the cost of formwork.

Приложение 5

Date: 2016-04-22; view: 740