4.1 EN 1998-1:2004
Coupling beams shall be designed and detailed in accordance with Clause 5.5.3.5:
The coupling of walls by means of slabs shall not be taken into account, as it is not effective.
The provisions for beam may only be applied to the coupling beams, if either one of the following conditions is fulfilled:
a) Cracking in both diagonal directions is unlikely. An acceptable application rule is:
VEd ≤ fctdbwd (4-1) where fctd is the design value of the concrete tensile strength.
b) A prevailing flexural mode of failure is ensured. An acceptable application rule is: l/h≥3.
If neither of the conditions in two above condition (a) and (b) is met, the resistance to seismic actions should be provided by reinforcement arranged along both diagonals of the beam, in accordance with the following (Figure 4.1):
a) It should be ensured that the following expression is satisfied:
VEd 2Asifydsinα (4-2)
where:
VEd is the design shear force in the coupling element (VEd = 2MEd/l);
Asi is the total area of steel bars in each diagonal direction;
α is the angle between the diagonal bars and the axis of the beam.
b) The diagonal reinforcement should be arranged in column-like elements with side lengths at least equal to 0.5bw; its anchorage length should be 50% greater than that required by EN 1992-1-1:2004.
c) Hoops should be provided around these column-like elements to prevent buckling of the longitudinal bars. The provisions of column hoops apply for the hoops.
d) Longitudinal and transverse reinforcement should be provided on both lateral faces of the beam, meeting the minimum requirements specified in EN 1992-1-1:2004 for deep beams. The longitudinal reinforcement should not be anchored in the coupled walls and should only extend into them by 150 mm.
4.2 ACI 318-08
Coupling beams shall be designed and detailed in accordance with Clause 21.9.7:
- The coupling beams with ln/h ≥ 4 shall satisfy the requirement of flexural members of special moment frames.
- The coupling beams with ln/h < 2 and with Vu > 4Acw(f’c)1/2 shall be reinforced with two intersecting group of diagonally placed bars symmetrical about the mid-span.
- The coupling beams not governed by two above categories shall be permitted to be reinforced either with two intersecting groups of diagonally placed bars symmetrical about mid-span or according to flexural members of special moment frames with satisfied requirements for longitudinal reinforcement, transverse reinforcement and shear strength requirements.
- The coupling beams reinforced with two intersecting group of diagonally placed bars symmetrical about the mid-span shall satisfy (a), (b), and either (c) or (d):
(a) Vn shall be determined by: Vn = 2Avdfysin 10Acw(f’c)1/2 (4-3) where is the angle between the diagonal bars and the longitudinal axis of the coupling beams.
(b) Each group of diagonal bars shall consist of a minimum of four bars provided in two or more layers. The diagonal bars shall be embedded into the wall not less than 1.25 times the development length for fy in tension.
(c) The detail requirements for the coupling beams are described in Figure 4.2, each diagonal element consist of a cage of longitudinal and transverse reinforcement, each cage contains at least four diagonal bars and confines a concrete core.
(d) The detail requirements for the coupling beams are described in Figure 4.3, this is second option for confinement of the diagonals to confine the entire beam cross section instead of confining the individual diagonals. This option can considerably simplify field placement of hoops, which can otherwise be especially challenging where diagonal bars intersect each other or enter the wall boundary.
When the coupling beams are not used as part of the lateral force-resisting system, the requirements for diagonal reinforcement may be waived.
4.3 Discussion
Firstly, in Vietnam standard of TCXD 198:1997 [7], Clause 3.4.2-Detail of shear wall and core wall, there was no any provision about design for the coupling beams, except only sentence of note and figure about detail for the coupling beams as shown in Figure 2.11.b, in which it may be used term of lintel beam for the coupling beams (Table 2.1). Detail of the coupling beams in TCXD 198:1997 also had no any note
about diagonal bars, hoops, development length, horizontal and vertical reinforcements, etc, everythings were not clear to design and detail. Since, problems of change for the coupling beams as mentioned above during design and construction process have been occurred for a long time. Until TCXDVN 375:2006 adopted, the coupling beams will be designed and detailed carefully according to standard’s requirements (Table 4.1). It is one reason in changing the coupling beams from design stage and then at construction site by designer, contractor, supervisor, owner…
Regardless of the diagonal bar ratio, ACI 318-08 requires at least 4 bars, while EN 1998-1:2004 do not have exact data. For the design and detail of diagonally oriented reinforcement in EN 1998-1:2004, it shall be followed EN 1992-1-1:2004, but for the hoops then its provisions in accordance with EN 1998-1:2004. It means that, the diagonal bars shall be designed and detailed by provisions for the non-earthquake resistant structures in EN 1992-1-1:2004, but the hoops for the diagonal bars shall be followed the earthquake resistant structures in EN 1:2004. Moreover, EN 1998-1:2004 also only have been presented a case for confinement of individual diagonals, and its figure shows not exact for arrangement of diagonal bars, it will be not constructed according to this figure (Figure 4.1) by coincide for the diagonal bars. There is difference between dimensions of group of the diagonal bars, ACI 318-08 requires b
≥ bw/2 and h ≥ bw/5 (out-to-out dimension), but EN 1998-1:2004 requires both b and h ≥ bw/2. Table 4.2 is brief summary on design and detail for the coupling beams based on codes of Vietnam, Europe and United States of America.
According to observations and analysis on experimental tests of conventional coupling beams (monolithic without any slits or keyways), four failure modes can be identified which are: Failure due to diagonal tension or diagonal compression, failure due to shear-slip or due to flexure and shear. It is considered that the weak connection
of monolithic coupling beams is the strength against shear. Therefore, placing diagonal bars or additional X-shaped steel in the plastic hinge zones at beam ends was suggested.
The ductility of the coupling beams with diagonal bars is good, but because instability of these bars out of its plane may occur, a minimum of thick for core wall or shear wall is required (Dajun et al. [45]). Figure 4.4 (Cheng [46]) shows seven specimens of coupling beams for experimental test, first two specimens, CB1 and CB2, are detailed in accordance with ACI 318-08 option 1 (confinement of individual diagonals) and option 2 (full confinement of beam section), respectively; other specimens are without diagonal bars, with diagonal bars, and three types of discontinuous diagonal bars at mid-span. With design of discontinuous diagonal bars at mid-span, force transfer mechanism through the coupling beams is not occurred effectively, shear wall structures are less stiffness that leads to increase failure possibilities, especially, priority cases of failure and damage will appear in the coupling beams at mid-span as shown in Figure 4.5.
Specimen CB3, CB4, CB5 with discontinuous diagonal bars at mid-span prematurely failed at mid-span when testing loads did not reach theoretical capacity, testing shear forces only were about 50% and less of nominal design shears. The displacement ductility ratio of specimens CB3 through CB5 are very small; while displacement ductility ratio of specimen CB2, approximately 6, is larger than CB7 (about 3). Since, specimen CB7 (without diagonal bars, but stirrup arranged to confine all the coupling beams) is used in low-to-moderate seismicity regions, specimen CB2 is option 2 (ACI 318-08) is designed in strong seismicity regions, and specimens CB3 through CB5 are not allowed.
By all above issues, it will be needed larger coupling beam width to have enough space for many reinforcements in the coupling beams, this issue seem to be unreasonable in practice by no engineers like to select large wall width by only reason
for code’s provisions of the coupling beams. Both codes require cage detailing of show that diagonally oriented reinforcement is effective only if the bars are placed with a large inclination. Therefore, diagonally reinforced coupling beams are restricted to beams having aspect ratio ln/h<4” (ACI 318-08 R21.9.7). For Figure 5.12 in EN 1998-1:2004 (Figure 4.1 in Chapter IV) also illustrates wrong arrangement for diagonal bars because they shall be staggered as figures in ACI 318-08.
For the longitudinal and transverse reinforcements in the coupling beam cross section, ACI 318-08 requires “Horizontal beam reinforcement at wall does not develop fy”, EN 1998-1:2004 presents “Longitudinal reinforcement should not be anchored in the couple walls and should only extend into them by 150mm”. In both codes, these longitudinal and transverse reinforcements are required to meet the minimum requirements and some other ones. In the second case of full confinement in ACI 318-08, the transverse reinforcement has key role to confine coupling beams cross section.
In practice, these longitudinal bars will be detailed as longitudinal bars in the wall, it means that they are extended into the wall with long length (see Figure 2.12, Chapter II) by more easy during construction. Consequently, what happen occurs when anchorage
them into wall. According to Lequesne et al. [47], fully anchored longitudinal bars are required.
The problem in bending of diagonal bars for enough development length in the wall mentioned in Chapter II (Figure 2.15): The development length of the diagonal bars anchored into the wall piers are usually calculated in tension condition. When this straight development length is not enough then they must be bent with angle as same as beam’s longitudinal bars anchored into the column core in beam-column joints.
Obviously, the anchored bend of diagonal bars is allowed completely but it is considered bending angle to ensure efficiency and easy to construct, for example, the bend angle should not be less than 90 degree. Both ACI 318-08 and EN 1998-1:2004 have no guidance for bending the diagonal bars when core wall section is shallow.
Probably, all experimental tests or research on coupling beams have been carried out by Dajun et al. [45] Harries et al. [48], Fortney [17], Fortney el al. [49] and even more early or lately, all of them did not mention bending angle of diagonal bars.
In order to improve the ductility of the coupling beams and prevent the brittle failure, many other studies of the coupling beams with the X-shaped steel, the coupling beams with a through-slit along the middle depth over the entire beam length, the coupling beams of this type, but also reinforced the ends of two small beams divided by a slit with X-shaped steel bars to strengthen the plastic zone against shear and to prevent shear-slip failure of the shear-compression zone along the normal cracks, and the coupling beams with slits and keyways as shown in Figure 4.6 (Dajun et al. [45]).
Cheng [50] researched on steel plate reinforced concrete coupling beams and it showed that the shear strength and ductility of ordinary reinforced concrete coupling beams could be significantly enhanced by using steel plate in replacing of conventional web reinforcements to resist very high shear stresses. Steel fibre reinforced concrete
coupling beams tested by Baczkowsk [51]. Addition of steel fibres significantly increases the inclination of the cracking angle and improves the capacity of energy dissipation, changes failure mode from very brittle to more ductile, which is a very important characteristic under earthquake loading. Steel fibre reinforced concrete should be treated as an addition to the concrete that improves its performance under the seismic loadings, especially so in moderate seismicity regions.
In high rise buildings, the coupling beams are necessary element in lateral force resisting structural system. However, in some specify cases, if analytical results show that design stiffness of structure system is enough to satisfy code’s requirements then the coupling beams can be dropped as this have more benefits to construct easily, for instance, the coupling beams should be replaced by slab or normal beam. All these will be depended on capacity of structural engineers. But there are some more important issues to consider carefully: Fracture failure at link between slab and core wall, punching condition, and shear force; and Clause 5.5.3.5(1)P said that “Coupling of walls by means of slabs shall not be taken into account, as it is not effective.” (EN 1998-1:2004).
The next future testing and studying for the coupling beams:
- Carrying out experimental tests for the coupling beams with difference bend angle of the diagonal bars.
- To continue to review the coupling beams with following solutions: Diagonally reinforced concrete coupling beams, steel coupling beams, concrete-steel composite, steel plate and steel firbe reinforced concrete coupling beams, excluding problems have been done by Dajun et al. [45], Harries et al. [48], Fortney [17], Forney et al. [49], Cheng [50], Baczkowsk [51] and some other researchers.
- Continuing research to propose typical coupling beams in buildings where size of the coupling beams is often fixed and less the changes.
- All research on the coupling beams will be considered to ensure to construct easily, increase quality for important structure resist lateral loads.
4.4 Brief summary
(1) Necessitating the use of diagonal bars in the coupling beams, detailing diagonal bars and changing content of diagonal bars in the coupling beams: No changes of detail for the diagonal bars are permitted, all changes or modifications are violated the terms of both codes.
The experimental studies and codes have been specified the reinforcement of diagonal bars in the coupling beams is needed, but the steel congestion in the coupling beams is the problem needed to consider further in future research to bring the best performance during construction.
(2) Discontinuity diagonal bars at mid-span: In the design or construction for the coupling beams, the diagonal bars do not extend to intersect at mid-span (Figure 2.12.b, 2.13.b) or diagonal bars is not provided (Figure 2.12.a): According to codes, as well as experimental tests have been carried out by Fortney [17], Fortney et al. [49], Cheng [46], these details shall be not acceptable. It is only used for the cases of small opening hole, and for the coupling beams accordance with code’s requirements, in which do not need to reinforce diagonal bars. However, one recommendation is that the option 2 in ACI 318-08 can be used for strong seismicity regions only, and specimen CB7 can be designed for low-to-moderate seismicity regions. Moreover, the diagonal bars shall be provided for the coupling beams with l/h2 (Cheng [46]).
(3) Anchored bend of diagonal bars: About bending angle of diagonal bars into the core wall (Figure 2.15), as beam-column joint behavior, they shall be completely bent with reasonable angle and direction. However, they shall be satisfied requirements on anchored bending angle and the development length in codes for tension or compression members, the development length in tension member is recommended.
Both EN 1998-1:2004 and ACI 318-08 do not have guidance for this case. Moreover, unfortunately, experimental tests are investigated not to mention the problem for bending angle for diagonal bars in the coupling beams. In case of anchored bend for diagonal bars, critical section is noted to calculate the development length. There is one solution for bending angle of the diagonal bars as shown in Figure 4.7.
(4) Anchorage of horizontal bars: Fully anchored longitudinal bars are required.
Figure 4.7 shows state-of-art solution for the coupling beams, this option is combined details in Figure R21.9.7.b (ACI 318-08) with experimental result by Lequesne et al.
[47].
(5) About problem of drop of the coupling beams and replace by slab: It is violated EN 1998-1:2004, Clause 5.5.3.5(1)P: “Coupling of walls by means of slabs shall not be taken into account, as it is not effective.” More specific requirement likes this provision is needed to add in standards and codes.
Finally, all issues of the coupling beams are very clear according to both codes and experimental tests. However, the width of core wall is problem because maybe it should be designed and detailed larger by requirements from the coupling beams.