Design and detailing of foundation elements

Foundation elements are normally made of concrete, even when the superstructure may consist of another structural material. Section 5 gives the design and detailing rules which apply to concrete foundation elements (footings, tie beams, foundation beams, foundation slabs and walls, piles and pile caps) even when the vertical elements founded through them are made of a different material. Section 5 also gives rules for the connection of concrete foundation elements to the vertical ones of the superstructure, applying only when the latter are also made of concrete.

Concrete foundation elements which are dimensioned for seismic action effects derived from either:

(1) the analysis for the design seismic action using a q factor less than or equal to the value of q for low dissipative behaviour (1.5 in concrete buildings, up to 2.0 in steel or composite buildings) according to clause 4.4.2.6(3) of EN 1998-1 or

(2) capacity design calculations according to clauses 4.4.2.6(2) and 4.4.2.6(4)-4.4.2.6(8) of EN 1998-1

are allowed to follow the simpler dimensioning and detailing rules applying to DCL (i.e. those of Eurocode 2 alone, plus the requirement to use steel of at least Class B), irrespective of the ductility class for which the superstructure is designed. The reason is that they are expected to remain elastic under the design seismic action (even when this is just due to the overstrength inherent in the q factor value for low dissipative behaviour in case 1 above). Clause 5.8.1 (3) Although choices 1 and 2 above are the only ones allowed for the verification of the foundation, Section 5 allows designing concrete foundation elements for energy dissipation, as in the superstructure. In that case they may be dimensioned for seismic action effects derived from the analysis for the design seismic action using the q factor chosen for the superstructure. They should also meet all the special dimensioning and detailing rules pertaining to the corresponding ductility class and applying to elements of the superstructure. This provision refers in particular to tie beams and to foundation beams, which should then be dimensioned in shear for a shear force derived from capacity design calculations, and should follow all the special rules for detailing of the longitudinal and transverse steel that aim at enhanced local ductility. Clause 5.8.1 (5) The best foundation system of a building from the point of view of earthquake resistance is commonly considered to be a box-type configuration consisting of:

(1) Wall-like deep foundation beams along the entire perimeter of the foundation, possibly supplemented by interior ones across the full length of the foundation system. These beams are the main foundation elements that transfer the seismic action effects to the ground. In dissipative buildings they are designed according to clause 4.4.2.6(8) as common foundation elements of more than one vertical member, normally by multiplying the design seismic action and its effects from the analysis by a factor of 1.4. In buildings with a basement, the foundation beams on the perimeter may also serve as basement walls.

(2) A concrete slab acting as a rigid diaphragm, at the level of the top flange of the foundation beams of the perimeter (as the roof of the basement, if there is a basement).

(3) A foundation slab or a grillage of tie beams or foundation beams, at the level of the bottom of the perimeter foundation beams.

Owing to its high rigidity and strength, such a system works as a rigid body. Thus, it minimizes uncertainties regarding the distribution of seismic action effects at the interface between the ground and the foundation system and ensures that all vertical elements undergo the same rotation at the level of their connection with this system, so that they may be considered as fixed against rotation at that level. Moreover, it ensures that the base of the superstructure is subjected to the same ground motion, smoothing out any differences in the motion over the foundation and filtering out any high-frequency components of the input.

Due to the high rigidity and strength of a box-type foundation system, that part of the columns within its height, as well as all beams within the foundation system (including those at the roof of the basement), are expected to remain elastic in the seismic design situation and hence may follow the simpler dimensioning and detailing rules applying to DCL (i.e. those of Eurocode 2 alone, plus the requirement to use steel of at least Class B), irrespective of the ductility class for which the building is designed.

Plastic hinges in walls and columns will develop at the top of a box-type foundation system (at the level of the basement roof slab). If the cross-section of a wall is the same above and below that level (as in interior walls that continue down to the level of the foundation system), that part of the height of the wall below the top the foundation system should be dimensioned and detailed according to the special rules of wall critical regions down to a depth below that level equal to the height of the critical region, ha, above that level. Moreover, as fixity of the wall at the level of the top of the foundation system is achieved via a couple of horizontal forces that develop at the levels of the top and bottom of the foundation system, the full free height of such walls within the basement should be dimensioned in shear assuming that the wall develops at the level of the top of the foundation system (basement roof) its flexural overstrength 7RdMRd (with 7Rd = 1.1 in buildings of ductility class M and 7Rd = 1.2 in those of DCH) and (nearly) zero moment at the foundation level.

The soffit of tie beams or foundation slabs connecting different footings or pile caps Clause 5.8.2 should be below the top of these foundation elements, to avoid creating a short column there, which is inherently vulnerable to shear failure.

Tie beams between footings and tie zones in foundation slabs should be dimensioned for the ULS in shear and in bending for the action effects determined from the analysis for the seismic design situation or via capacity design calculations, and to a simultaneously acting axial force (tensile or compressive, whichever is more unfavourable) equal to a fraction of the mean value of the design axial forces of the connected vertical elements in the seismic design situation. This fraction is specified as equal to the design ground acceleration in g, aS, multiplied by 0.3, 0.4 or 0.6 for ground type B, C or D, respectively. The purpose of the additional axial force is to cover the effects of horizontal relative displacements between foundation elements not accounted for explicitly in the analysis for the seismic design situation. It may be neglected for ground type A, as well as in low-seismicity cases (recommended as those with aS < 0.1) over ground type B.

The minimum cross-sectional dimensions and the minimum longitudinal reinforcement ratio of tie beams or foundation beams and of tie zones in foundation slabs used instead of tie beams are Nationally Determined Parameters. If tie beams are designed for energy dissipation (i.e. if they are dimensioned for the ULS in bending and in shear for seismic action effects derived from the analysis using a q factor value higher than that corresponding to low-dissipative structures), then they should meet also the minimum reinforcement requirements of the corresponding ductility class.

The connection of a foundation beam or a foundation wall with a concrete column or wall Clauses 5.8.3(1), is essentially an inverted-T or knee 'beam-column joint'. Therefore, it should be dimensioned 5.8.3(4)

and detailed according to the rules for beam-column joints of the corresponding ductility class. This implies that the transverse reinforcement placed in the critical region at the base of the column or the wall should also be placed within the region of its connection with the foundation beam or wall, except for interior columns founded at the intersection of two foundation beams with width at least 75% of the corresponding dimension of the column. In that case the horizontal reinforcement is placed in the connection at a spacing which may be double that at the column base, but not more than 150 mm. It is noteworthy that the horizontal reinforcement at the connection of a concrete wall with a foundation beam or wall is also specified through reference to the transverse reinforcement in the critical regions of DCM columns. However, as the rules are essentially the same as those for the transverse reinforcement in boundary elements within the critical region of ductile walls, the horizontal reinforcement to be placed in the connection of a wall and a foundation beam (or wall) should have the same diameter and spacing as the peripheral ties of the boundary elements of the wall critical region above, but it should extend over the entire periphery of the horizontal section of the connection region. Clauses 5.8.3(2), In addition to being subject to the prescriptive detailing of the previous paragraph, in 5.8.3(3) buildings of DCH the connection region of a foundation beam or wall with a concrete column or wall should be explicitly verified in shear. The design horizontal shear force to be used in this verification, V-hi, should be established as follows:

8 If the foundation beam is dimensioned on the basis of seismic action effects derived from capacity design considerations (i.e. in practice for the seismic action effects from the analysis for the design seismic action multiplied by 1.4), then Vjhd may be determined from the analysis for the design seismic action. Because this analysis does not directly provide seismic action effects for the joints, Vjhd may conservatively be estimated as the design value of the flexural capacity at the base section of the column or wall, MRd, divided by the depth of the foundation beam, hb. • If the foundation beam is dimensioned on the basis of seismic action effects derived directly from the analysis for the design seismic action, then Vjbd itself should be determined via capacity design calculations, namely through equation (D5.21), using as Asbl and Asb2 the areas of the top and bottom reinforcement in the foundation beam, respectively. This approach is never unconservative (on the unsafe side) for the connection region.

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  • phil
    How to design box type foundation with eurocode 8?
    8 years ago

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