Indicativ p85-96 - reinforced concrete structural walls

INDICATIV P85-96

REINFORCED CONCRETE STRUCTURAL WALLS

A). CALCULUS OF THE TRUCTURAL WALLS SECTIONS

1. Plastic zones

There are considered plastic zones of the structural walls the following situations:

for the coupling beams, the whole span;

for the structural walls (isolated or coupled), the zone having the length l_p measured from their base:

l_p = 0.4h + 0.05H

In the case of multistory buildings, this dimension is upward averaged to a whole number of stories, if the limit of the plastic zone exceeds the height of a level with more than 0.2H_level. If not, it is downward averaged to a whole number of stories.

The zone from the base of the wall limited in this way is called Zone A.

The rest of the wall, less loaded and with reduced design demands compared with zone A, is called Zone B.

2. The dimensioning values of the sectional efforts.

2.1. The dimensioning values M of the bending moments

The values are determined with relations from the figure:

a) In over structure (above the infrastructure or foundation):

M = k_MwM_s wM_s,o

b) In infrastructure:

M = 1,5wM_s

where

M_s = bending moment from the design seismic loading; at the basement level has the value M_{s, o};

w = .. between the overturning moments;

M_{o, cap} = computed at the basement level of the structure, associated to the strength capacity of the structural wall, cantilevered or coupled, and the value of the overturning moment;

M_o = from the design seismic loading (see the figure):

M_i,cap = the capable moment at the base of cantilever wall i;

N_i = the axial effort at the base of the cantilever wall i, produced by the horizontal forces, in the case of the plastification of the coupling beams extremities;

L_i = the distance from the axis of the cantilever wall i to the reference point with respect to whom the moments are computed;

k_M = correction coefficient of the walls bending efforts.

2.2. The dimensioning values Q of the shear forces

In the case of the structures for which the seismic forces are taken over in totality by the structural walls, the value of Q is determined with the relation:

1,5Q_s Q = k_Q w Q_s 4 k_Q Q_s

where

Q_s = the shear force from the design seismic loading;

k_Q = correction coefficient of the shear forces:

k_Q = 1 + 0,03n

n = the number of stories

2.3. The value of the design shear force for the coupling beams

It is determined with:

where

the absolute values of the capable moments in the sections at the extremities of the coupling beams, from both senses of action of the moments, established on the base of the design strength R_a of the reinforcement; for the value of M_cap^sup, the reinforcement contribution in the active zone of the floor will be taken into consideration;

l_r = l_o = the span of the coupling beam.

2.4. The dimensioning axial forces from the coupling walls

The dimensioning axial forces from the coupling beams are estabished on the base of the wall equilibrium in the situation of plastified coupling beams, but without the 25% increased value of the bending strength capacity ,considered in the above relation.

3. The computation of the longitudinal and transversal reinforcement of the structural walls

3.1. The computation of the longitudinal reinforcement

The computation of eccentrical compression (tension) of the structural walls is made according to the prescriptions from the STAS 10107/0-90 code.

3.2. The computation of the structural walls for the shear force

The computation for the shear force is made in inclined sections and horizontal sections at the casting level.

a) The computation in inclined sections

In the case of the structural walls with the ratio between the elevation height and the section height H/h > 1, the dimensioning of horizontal reinforcement A_{a o} for the shear force in inclined sections is based on relation:

Q Q_b + 0,8 A_ao R_a

where

A_{a o} = the sum of the horizontal reinforcement sections intersected by a 45^o inclined crack, including the belt beams reinforcement and the continuous reinforcement from the active zone of the floor (include three floor heights on both sides of wall), if the crack crosses the floor;

Q_b = the shear force taken over by the concrete, with the values:

Q_b = 0,3bhs_o 0,6bhR_t for the zone A of the wall;

Q_b = bh(0,7R_t + 0,25s_o for the zone B of the wall.

the average compression stress in wall section. When is tension, is taken with minus sign in zone B and zero in zone A.

b) The computation in horizontal sections at casting levels.

The dimensioning is made according to the prescriptions from the STAS 10107/0-90 code.

4. The computation of the reinforcement from the coupling beams

The computation of the longitudinal reinforcement is made according to the prescriptions from the STAS 10107/0- 90 code.

4.1. Coupling beams reinforced with horizontal bars in stirrups

The transversal reinforcing is determined from condition that these ones have to take over the whole design shear force, according to relation:

where:

A_ae = the necessary area of the section of one stirrup;

a_e = the distance between the stirrups;

n_e = the number of the stirrup legs.

4.2. Coupling beams reinforced with inclined bars

The area A_ai of the inclined reinforcement on each diagonal is determined with the relation:

where = the inclination angle of reinforcement.

B) CONSTRUCTIVE REQUIREMENTS

1. The wall reinforcing. General provisions

1.1. The reinforcement overlapping

a) It is recommended that the vertical reinforcement to be realized without hooks.

b) For the reinforcement with independent bars, in the potential plastic zone (zone A), the minimum overlapping lengths are given in the table.

For the zone B the minimum overlapping lengths are with 10d less than those from the table. Also, in the zone B it is not necessary the overlapping by welding of the reinforcement with d 16(20) mm.

1.2. The reinforcement anchorage

a) Horizontal bars from the belt beams and the horizontal independent bars from the field reinforcement at intersections in T or L shapes.

Bars with.: l_a = 40d (OB 37, PC 52, PC60)

Bars without:  l_a = 40d (PC 52, PC60)

l_a = 60d (OB 37)

b) Horizontal bars from the coupling beams. According STAS 10107/0-90

c) Vertical bars from walls anchored in foundations. According to STAS 10107/0-90

d) Vertical boarding bars of the openings which are anchored in a wall at an inferior level. l_a will be determined such that to contain a corresponding number of vertical bars from the current reinforcement of the inferior wall, but minimum 2.00 m.

The mesh, which form the continuous reinforcement of the walls, will be connected with hooks in order to insure their position during the concreting.

Usually, there will be at least:

4 hooks/m² for bars with d < 8 mm;

6 hooks/m² for bars with d > 8 mm.

2. Field reinforcement of the structural walls

2.1. The strength reinforcement

It is stipulated in the following cases:

in zone A

in zone B, when Q > Q_b;

in short walls (H/h < 1)

In zone A, regardless the type of wall, the minimum reinforcement percentages are those from the table.

Outside the wall A, the minimum reinforcement values for zone F will be adopted.

The minimum diameter:

6 mm for horizontal reinforcement

8 mm for vertical reinforcement

The maximum distance:

350 mm on horizontal direction

250 mm on vertical direction

2.2. The constructive reinforcement

It is realized from two nets 5 at 200 mm, from STNB, one at each part of the wall, or with other steel reinforcement having equivalent diameters.

For the wall core around the stairs, on the whole length, and for the last level in all cases, horizontal reinforcement will be added having the following minimum percentage:

0.25% for OB 37

0.20 for PC 52 or PC 60

3. Local reinforcement of the vertical elements

3.1. The reinforcement of the zones from the structural walls extremities

In the zones from the structural walls extremities, on the surfaces indicated in the figure1, for lamellar sections, in the figure 2, for sections having boundary elements, and in figure 3 for coupled walls sections, the reinforcement is realized with cages of the same type as those used for columns.

The minimum vertical reinforcement percentage of this zones reported to their area are showed in the table:

The local reinforcement will have to obey, from the point of view of distribution and the minimum number of bars, the details from the figure:

The concentrated reinforcement together with the vertical reinforcement situated in the web and the flange of the walls, including that in the intermediary intersections, have to insure the wall section a superior bending strength capacity compared to the cracking moment M_f of the section, determined with:

M_f = Nr_s + 0.5 c_pl W_f R_t

where

r_s = distance from the center of gravity of the section to limit of the central core, situated on the same part with the eccentrically force N;

W_f = the cracking resistance modulus of the section, computed considering the tension zone completely plastificated;

c_pl = coefficient which takes into consideration the partial plastification of the tensioned zone of the section; see table 15, STAS 10107/0-90.

Stirrups

Minimum diameter: 6 mm and d/4 (d = the minimum diameter of the vertical bars of the A_a reinforcement).

Maximum distances:

zone A:

125 mm, but no more than 10d, for design seismic zones A - E;

150 mm in design seismic zone F;

zone B:  - 200 mm, but no more than 15d.

The stirrups of the cage will be made such that their area have at least the same strength capacity as the horizontal reinforcement from the adjacent fields. In the case when this reinforcement is interrupted in the cage, it is properly overlapped with the cage stirrups.

3.2. The confining reinforcement in the compressed zones

If the height of the compressed zone of the sections, when the strength capacity is reached, is higher than the limit value (x > x_lim), a special confining reinforcement in the compressed zone will be realized on a length of at least x/2.

The confining reinforcement quantity A_{a o}, in each direction is computed with relation:

where

a_e = axial distance between sets of confining stirrups;

c = dimension of the central core between the confining stirrups, measured perpendicular to the stirrups braces;

Also, in the A_{a o} reinforcement can be considered the horizontal reinforcement of the web, if this one is bent after the vertical bars and properly anchored.

In the zones where the longitudinal reinforcement percentage takes over the value 2/R_a (N/mm²), supplementary measures will be taken such that to avoid buckling of the bars situated in the potential plastic zone. The confining reinforcement A_{a o} can have this role. In these zones it is recommended that the transversal connection by stirrups and hooks of the vertical bars with diameter d > 16 mm to be done with a maximum distance of 6d.

3.3. The reinforcement of structural walls intersections

The interior intersections are reinforced with cages with 2 crossed stirrups, which make the connection with the horizontal reinforcement of the walls.

The maximum distance between the stirrups: 200 mm.

The minimum vertical reinforcement of the intersection zones: 4

3.4. The reinforcement around the openings

Around openings with small dimensions reported to the wall dimensions, and which do not influence significantly the general ensemble behavior, a constructive reinforcement will be realized on each side with at least two bars 10 mm and having the section equivalent to the reinforcement interrupted by the opening.

3.5. The reinforcement of the walls and floors intersections

A belt reinforcement having at least four bars will be made along the width of the wall.

Minimum diameter of the bars: 10 mm.

4. The reinforcement of the coupling beams

4.1. The reinforcement with longitudinal bars and vertical stirrups

a) The longitudinal bars resulted from the bending moment dimensioning, placed at the superior and inferior part of the section

Minimum diameter of bars: 10 mm.

Recommended steel types: PC 52, PC 60.

b) Intermediary longitudinal bars placed on lateral sides

Minimum diameter of bars: 8 mm.

Minimum reinforcement percentage reported to the bh_r section:

0.20% for seismic zones A - E;

0.12% for seismic zone F

c) Vertical stirrups

Minimum diameter of bars: 6 mm.

Minimum transversal reinforcement percentage: 0.20%

Maximum distance between stirrups: a_e 8d; a_e 150 mm (d = longitudinal reinforcement bars diameter placed at superior and inferior part of section).

4.2. The reinforcement with crossed inclined bars

Anchorage length of the inclined bars: > 60d;

Distance between stirrups: > 6d (d= diameter of the inclined bars);

Minimum diameter of the stirrups: d/4;

Minimum horizontal reinforcement percentage: 0.2%;

Minimum transversal reinforcement percentage (with stirrups): 0.15%.