Special rules for concrete systems with masonry or concrete infills

Section 4 of EN 1998-1 contains special rules for the analysis and design of frame (or frame-equivalent) concrete buildings and of unbraced steel or composite buildings with non-engineered masonry infills (see Section 4.12 of this guide). These rules are mandatory only for buildings designed for DCH. If the building is designed for DCM or DCL, the rules of Section 4 are considered to serve only as a guide to good practice.

Section 5 contains additional rules for concrete buildings with infills, which apply to buildings designed for either DCH or DCM, (but not for DCL), irrespective of the structural system. The objective of these rules is to protect concrete buildings from the adverse local effects of infills.

The potential adverse local effects of infills are mainly from two sources:

• damage or even failure of columns in contact with strong infills over their full height, due to non-uniform and/or unbalanced contact conditions, or

• a reduction in the clear height (and hence in the effective shear span) of columns due to contact with (and restraint by) infills over part of the full height; the resulting 'short' or 'captive' column is prone to flexural/shear failure or a pure shear failure dominated by diagonal compression.

Clause 5.9(1) Part of an infill panel may be dislocated by failure or heavy damage, exerting a concentrated force on the adjacent column. The stronger the infill, the larger the magnitude of this force and the higher the likelihood of local column failure. Infill panels are more likely to fail or suffer heavy damage at the ground storey, as there the shear force demand is largest. For this reason, in buildings with masonry or concrete infills, the entire length of the columns of the ground storey is considered a critical region and subject to the corresponding special detailing and confinement requirements, to be prepared for local overloading by the failed infill panel at any point along its height.

Clause 5.9(3) Unbalanced contact conditions may take place in columns with a masonry infill on only one side (e.g. corner columns). The entire length of such columns is considered a critical region and subject to the associated special detailing and confinement requirements.

Clause 5.9(2) The lateral restraint of a column due to the contact with the infill over part of its full height is normally sufficient to cause the plastic hinge to develop in the column at the elevation where the infill is terminated, instead of the end of the column beyond the contact with the infill. This may be the case even when at this latter end the beams are weaker than the column and equation (D4.23) is satisfied. So, Section 5 requires calculation of the design shear force of the 'short' or 'captive' column through equation (D5.12), with:

(1) the clear length of the column, /cl, taken equal to the length of the column not in contact With the infills

(2) the term min(...) taken equal to 1.0, at the column section at the termination of the contact with the infill wall.

Moreover, as (1) the clear length of the column may be short and (2) the exact location and extent of the potential plastic hinge region near the termination of the contact with the infill wall is not clear and may well extend into the region of the column in contact with the infill, it is a requirement to:

• place the transverse reinforcement necessary to resist the design shear force not just along the clear length of the column, /c], but also along a length into the column part in contact with the infills equal to the column depth, hc, within the plane of the infill

9 consider the entire length of the column as a critical region and provide it with the amount and pattern of stirrups specified for critical regions.

This additional transverse reinforcement will increase the nominal shear resistance of the 'captive' column over its full length, beyond the design shear force for which it has been verified, and will enhance its deformation capacity for any potential location of the plastic hinging. This may partly compensate for the lack of a special rule in Eurocode 8 for the calculation of the nominal shear resistance of columns with a low shear span ratio ('squat columns'), regardless of their reduced cyclic shear resistance as controlled by failure of the concrete along the diagonal(s) of the column in elevation. In fact, cyclic test data from 44 columns with shear span ratio, LJhc, less than or equal to 2 that have failed by shear compression suggest the following expression for their shear resistance as controlled by failure of the concrete (units: meganewtons and metres):

Equation (D5.53) is the counterpart of equation (D5.47) for squat columns; all variables in it are defined as in equation (D5.47), except (1) the internal lever armz, which is taken here to be equal to z = d-d' and (2) 6 in the last term, which is the angle between the axis of the column and its diagonal in elevation (tan 9 = hJ2Ls). Equation (D5.53), proposed by Biskinis et al.,69 has been adopted in Annex A of EN 1998-352.

If the clear length of the column, /cl, as specified in point 1 above, is short, then the design shear force may be so large that it may be difficult to verify the column for it, especially as the critical shear resistance may be controlled by shear compression (cf. equation (D5.53)) and cannot be increased through transverse reinforcement. Although designation of such a column as 'secondary seismic' (cf. Section 4.10) may seem a convenient way out of this predicament, it is far more sensible to attempt to solve the problem through a change of the geometric conditions by either:

(1) changing the configuration of the infills and their openings to remove the partial-height contact of the column with the infill or increase the clear length of the column, ld, beyond this contact or

(2) changing the cross-sectional dimensions of the column.

Option 2 should be exercised to reduce the size of the column, rather than increasing it:

8 if the shear span ratio, LJhc, of the column increases above 2 (or, preferably, 2.5) its behaviour in cyclic shear will not exhibit the special vulnerability and low dissipation capacity which characterizes short columns • the decrease in the cross-sectional dimensions will reduce the design shear force from equation (D5.12) (by reducing the design values of the flexural resistance of the column, MRdc ¡, i = 1,2) more than it will reduce the nominal shear resistance, helping both the verification as well as the physical problem.

Reinforcement placed along both diagonals of the clear length of the short column within the plane of the infill is very effective in increasing its energy dissipation and deformation capacity. Placement of such reinforcement, in addition to or instead of the conventional transverse reinforcement of the column, is another viable option. This reinforcement may be dimensioned to resist at the same time the design shear force from equation (D5.12) as well as the design bending moments at the end sections of the short column, in accordance with the relevant rules for coupling beams in coupled walls. Placement of such reinforcement and its dimensioning to resist the full value of the design shear force is mandatory, if the clear length of the column, ld, is less than 1.5hc (corresponding to a value of the shear span ratio, LJhc, less than 0.75).

To prevent shear failure of columns with a masonry infill on only one side, a length, lc, at Clause 5.9(4) the top and the bottom of the column over which the diagonal strut force of the infill may be applied, should be verified in shear for the smaller of the following two design shear forces:

(1) the horizontal component of the strut force of the infill, taken as equal to the horizontal shear strength of the panel, as estimated on the basis of the shear strength of bed joints (shear strength of bed joints multiplied by the horizontal cross-sectional area of the panel, bw, multiplied by the clear panel length Lbn)

(2) the shear force computed from equation (D5.12), applied with clear length of the column, ld, taken as equal to the contact length, lc, and the parentheses in the numerator equal to twice the design value of the column flexural capacity, 2MRd c.

In case 2 the contact length should be taken as equal to the full vertical width of the diagonal strut of the infill. This is consistent with the calculation in case 1, which conservatively assumes that the full strut force is applied to the column. It is also closer to the reality at the top of the column, as there the joint between the top of the infill and the soffit of the beam may be open due to creep of the masonry or concrete infill.

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