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Strut and Tie Modelling of Reinforced Concrete Deep Beams

Ajibola Ibrahim Quadri at Yokohama National University

Abstract and Figures

Numerical analysis of the performance of reinforced concrete (RC) deep beam subjected to static and fixed-point pulsating loading at the midpoint has been investigated. Three-dimensional nonlinear finite element model using the Strut and Tie approach was adopted. The damage level under the influence of the applied fixed pulsating loading is higher than the static applied loading, hence early crack was observed because of the stepwise loading in the form of vibration. Although the Strut and Tie approach gave a good estimation of the resistance capacity of the beam, the beam undergo high shear damage when subjected to these two types of loading. Material strength properties, applied loadings and cross-sections adopted are some of the factors that affect the performance of the deep beam.

Beam dimension and strut and tie layout A beam of 1m, a width of 400mm and the total length of 2.4 m were adopted. The beam self-weight is calculated as part of the applied load, the solution for the Strut and tie (STM) is given as; The reaction regarding Figure 3, í µí± í µí°´=µí°´= í µí± í µí°µ = 1000 í µí±˜í µí± The shear span to depth ratio of this beam is less than 2, therefore, the beam is considered as the disturbed region (D region). Since the beam is symmetrical, the developed STM layout in Figure 3 consists of the strut in compression and the tie in tension. The width of the bearing plate is assumed as 200 mm, the upper nodes are usually located at a distance, c, equal one-quarter of the width of the bearing plate. Which is 50 mm. hence, the distance is assumed as 50mm. The length of the diagonal strut C1 is;

Beam dimension and strut and tie layout A beam of 1m, a width of 400mm and the total length of 2.4 m were adopted. The beam self-weight is calculated as part of the applied load, the solution for the Strut and tie (STM) is given as; The reaction regarding Figure 3, í µí± í µí°´=µí°´= í µí± í µí°µ = 1000 í µí±˜í µí± The shear span to depth ratio of this beam is less than 2, therefore, the beam is considered as the disturbed region (D region). Since the beam is symmetrical, the developed STM layout in Figure 3 consists of the strut in compression and the tie in tension. The width of the bearing plate is assumed as 200 mm, the upper nodes are usually located at a distance, c, equal one-quarter of the width of the bearing plate. Which is 50 mm. hence, the distance is assumed as 50mm. The length of the diagonal strut C1 is;

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Al-Nahrain Journal for Engineering Sciences NJES 23 (3)306-312 , 2020

http://doi.org/10.29194/NJES.23030306

NJES is an open access Journal with ISSN 2521- 9154 and eISSN 2521-9162

This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License

306

Strut and Tie Modelling of Reinforced Concrete Deep Beams Under

Static and Fixed Pulsating Loading

Ajibola Ibrahim Quadri

Authors affiliations:

1) Department of Civil

Engineering, The Federal

University of, Akure-Nigeria.

dgeneral1201@gmail.com

Paper History:

Received: 22nd April 2020

Revised: 29th June 2020

Accepted: 14th Aug. 2020

Abstract

Numerical analysis of the performance of reinforced concrete (RC)

deep beam subjected to static and fixed-point pulsating loading at the

midpoint has been investigated. Three-dimensional nonlinear finite element

model using the Strut and Tie approach was adopted. The damage level

under the influence of the applied fixed pulsating loading is higher than the

static applied loading, hence early crack was observed because of the

stepwise loading in the form of vibration. Although the Strut and Tie

approach gave a good estimation of the resistance capacity of the beam, the

beam undergo high shear damage when subjected to the se two types of

loading. Material strength properties, applied loadings and cross-sections

adopted are some of the factors that affect the performance of the deep

beam.

Keywords: Deep Beam, Fixed Pulsating Load, Static Load, Strut and Tie Model

1. Introduction

Reinforced concrete (RC) structures are subjected

to various deterioration such as concrete spalling,

cracks, large deformation, rebars rupture, and

collapse during their service time. This is caused by

environmental and man-made factors; aging of the

structures, reinforcing bar corrosion, earthquakes,

blasts, overloading, etc. [1]. The behavior of some RC

members is dominant under shear failure while some

experienced flexural failure or bending. Some parts of

RC structures are overestimated with certain accuracy

during design, while other parts are designed using

the rules of thumb or based on experience. However,

all parts of structures are important and must be

treated accurately because of the nonlinear behavior

of concrete [2]. When the span-to-overall depth ratio

of a beam (l/d) equal to 4 or less, or the shear span-

to-overall depth ratio of a beam (a/d) is equal or less

than two, the beam is termed a deep beam. It is often

loaded at the face and supported at the opposite face.

Deep beams are qualified by having a shear span-

depth ratio (a/d) relatively small Figure 1, [3] thus,

their behavior is dominant under shear rather than

flexure and their capacity depends on the mode of

application of load and supports condition. Many

investigations have been conducted on the behavior

of a simply supported RC deep beam under static

loading [3,4], however, some RC deep beams are

under the influence of pulsating loading such as the

vibrating machine. Their behaviors are different when

compared to the static loading [5].

Figure (1): Common types of struts and tie

1.1 Research Significance

Deflection of RC deep beam under static loading

is always apparent and allows users to evacuate

before the total failure, however, little or no warning

is given under the pulsating types of loading. Due to

the heterogeneous constituents of concrete, load

stress generated under pulsating load is always lower

than that of the static stress capacity, thus, crack

initiation to propagation may not be apparent before

failure. By using the structs-and-ties approach, the

bond behavior of the RC beam can easily be analyzed

under continuum model. The results of the past

NJES 23 (3)306-312 , 2020

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307

experimental investigations show that the bond-zone

response of RC structure is highly nonlinear and

always affected by the stress-strain state of the

structure [5]. Hence, strut and tie approach is

proposed to model the bond strength in the RC deep

beam

2. Struct s and Ties Consideration

The truss analogy was introduced by Ritter [6],

and Morsch [7] more than a century ago, to show the

emergence of internal stresses in a cracked beam,

which is the basis for the design of concrete beams.

This method was improved and extended to be struts

and ties by Leonhardt [8], Rusch [9] and Kupfer [10].

Thereafter, Collins and Mitchell, [ 11] further

investigated the deformation of the truss model and

came up with a refined approach for shear and

torsion. Demerdash, [12 ] considered a nonlinear

finite element approach to design the behavior of

deep beam, it was reported that cracking patterns,

deflections, failure mode, and stress-strain

distributions, cannot be determined with the strut-

and-tie model.

Strut and Tie models (STM) are one of the

approaches to describe stress growth and distribution

developed in a continuum structural field. The action

of external loading resulting in flexure, shear,

compression, torsion is initially borne by the material

constituents in the microscopic level, the overall

effect is visible as a failure because once the material

is completely in the plastic region, relatively low

additional resistance to increase loading is exhibited.

Strut and Tie models are basically a truss

analogy that essentially depends on the limit analysis

of the lower bound theorem of collapsed load [13]. In

this case, only the equilibrium and the yield criterion

are satisfied with stress generalization. Hence, in

reinforced concrete structures, members subjected to

compression termed as Struts are the concrete. The

strut transfers stresses from node to other nodes and

may be reinforced as structural elements or a typical

wall. While the members in tension consist of the

steel reinforcement are the Ties that transfer tensile

stresses to the nodes. On many cases, transverse

reinforcement is provided to prevent the splitting of

the RC caused by transverse tension due to the high

compressive stress generated by the struts. The shape

of the strut depends upon the stress path from which

the strut emerges, and details of any tension joined to

the tie, [14]. Struct are classified into three as shown

in Figure 1, Prismatic structs are mainly adopted to

model the compressive stress block of a beam

member. When the section at the end of the strut is

well defined but the rest of the struct is not confined

to a specific portion of the member, Bottle-shaped

strut is adopted to model the section, Compression

fan strut emerges when stresses flow from a large

source to a smaller source.

ACI 318R-19 [1 5] has been adopted for the

analysis of the strut and tie model in this study. The

strength of concrete in compression depends on the

multi-ax ial state of stress and the disturbance from

reinforcement and cracks. The effective compressive

strength of the concrete strut is given in Eq. 1 as;



   

(1)

 is the cylindrical concrete compressive

strength and  

for the cube concrete

compressive strength. is the effective factor for

concrete strut which accounts for the stress

conditions, geometry and the crack angle surrounding

the strut. The values of for various conditions

adopted for this investigation according to the ACI

318R-19, [15] is presented in Table 1. The tie cross

section area,  , is taken as constant along its length

and is obtained from the tie force and the yield stress

of the steel, . The nominal strength of a tie Ft is

expressed in Eq. 2 as;

   (2)

Table (1). Strut and Node Efficiency Factor

According to ACI 318M-11

Strut and Node Efficiency

Struts with uniform cross-section

over its length

Bottle-shaped strut with

reinforcement

Bottle-shaped strut without

reinforcement

Struts in the tension member

The compressive strength of concrete at the

nodal zone depends on the tensile straining from tie

intersection, confinement provided by compressive

forces and the confinement by the transverse

reinforcement. To distinguish between the

confinement condition and straining for the nodal

zone, Figure 2 presents the node classification.

C-C-C is the nodal zone bounded by

compression strut only,

C-C-T is the nodal zone bounded by

compression struts and a tension tie,

C-T-T is the nodal zone bounded by a

compression strut and two tension ties, and

T-T-T is the nodal zone bounded by tension

ties only.

Figure (2): Nodes Classification

The effective compressive strength of concrete in

a nodal zone, 

is given in Eq. 3 as;



   

(3)

NJES 23 (3)306-312 , 2020

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308

is the effectiveness strength factor of a nodal zone

as given in Table 1.

The nominal compressive strength of a nodal zone,

is given in Eq. 4 as;

  

(4)

is the minimum of the area of the nodal zone

where the force acts.

3. A numerical scheme for the beam

verification

To determine the reinforcement for the simply

supported deep beam in Figure 3, an assumed

factored load of 2000kN was used, a concrete

cylindrical strength, , steel yield stress



Figure (3): Beam dimension and strut and tie layout

A beam of 1m, a width of 400mm and the total

length of 2.4 m were adopted. The beam self-weight

is calculated as part of the applied load, the solution

for the Strut and tie (STM) is given as;

The reaction regarding Figure 3,  



The shear span to depth ratio of this beam is less

than 2, therefore, the beam is considered as the

disturbed region (D region). Since the beam is

symmetrical, the developed STM layout in Figure 3

consists of the strut in compression and the tie in

tension. The width of the bearing plate is assumed as

200 mm, the upper nodes are usually located at a

distance, c, equal one-quarter of the width of the

bearing plate. Which is 50 mm. hence, the distance is

assumed as 50mm.

The length of the diagonal strut C1 is;

     

The force in strut  





The force in    

 

 

The angle between and T,    

 

 

3.1 Effective concrete design strength of

struts and nodes

The effect of concrete design strength of the

struts and nodes are calculated using the cylindrical

concrete compressive strength as given in Eqs. 1 and

3 respectively.

The strut strength for the inclined strut by

adopting   for the bottle-shaped is;

 

  

The strut strength for the inclined strut by

adopting   for the prismatic shape is;



  

The effective design strength of the node using

the cylindrical compressive strength for various nodes

classification and adopting equivalent in Table 1,

can be calculated as;

At A, the node component is C-C-T thus,



  

At B, the node is C-C-C, 

 



The bearing stress generated from node B is given as



     

   

For the section at the interface between strut C 2

and the node, the design strength should be the

smaller of the node strength and the strut strength,

both of which are the same in the case of 25.5 MPa.

The required width of strut C2 is calculated as

   

  

  

The difference between the assumed width of

C2 =100 mm and the required width is on the safe

side and is not significant, there is no need for

dimension modification.

NJES 23 (3)306-312 , 2020

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309

The required width for the strut C1 is calculated

as;

    (5)

    



The corresponding stress is given as;



 (6)



      

    

The stress generated at node A is given as 

 

   

     

3.2 Required Reinforcement

Since the strut joining nodes A and B has a

bottle-shaped stress field, transverse reinforcement of

the strut is required to resist a total force.

Strut C1 requires transverse reinforcement as;

 

   





The total required reinforcement perpendicular

to the strut is;



 

The total length of C1 is calculated as 1204.16

mm, hence the required transverse reinforcement is



    , hence, a 10 mm

diameter bar of 100 mm spacing is provided.

Tie T:

The reinforcement required to resist the force of this

tie is;



 

hence,  is provided.

4. Finite Element Model and Loading

Three-dimensional nonlinear finite element model

of the RC deep beam was adopted, the option is to

present the hexagonal behavior of the nodes in an

element. The meshing of the individual RC element

has been achieved with the Concrete Model of 3-

Dimension software package which allows the

simulation of nonlinear response of concrete

structures, the software allows the two materials

(concrete and steel) to have a common node with a

common element bonded together. The beam

supports are made of elastic material which is simply

supported. Figure 4 shows the model of the deep

beam with the calculated reinforcing bar detailing.

The model ensures proper bonding between the

reinforcement and the concrete.

Table 2 shows the material strength properties of

the concrete, reinforcement and the elastic support.

Two kinds of loads; a static load at the top of the

beam, was applied on each node with its magnitude

calculated, and a pulsating load (this was applied by

loading and unloading on each beam) as shown in

Figure 4. The pulsating load was step-wisely applied

on the nodes with the loading and unloading within

0.003 seconds, by adopting 5 time-step.

Numerical FEM investigation of the of the beam

was carried out with the help of the Concrete Model

of 3-Dimension. The beam was discretized into 894

elements, each element has the same dimension, and

1420 nodes.

Figure (4): 3-dimensional FE model of Deep beam with rebar detailing

Table (2). Material Strength and Properties

Concrete Compressive

strength (MPa)

Strain softening

coefficient

4.1. Cracks Delineation

The crack formed in the deep beam was

evaluated in terms of the localized principal stresses

and strains generated in the discretized critical

element along the flexural cracks as expressed in Eqs.

7 and 8. Localized damage was evaluated using the

combined stress-strain effect from the reinforced

concrete element of the deep beam. Once the load

reaches the ultimate value, the maximum yield strain

is simultaneously reached hence further loading does

not have an impact on the failure of the beam.

NJES 23 (3)306-312 , 2020

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310

 

     (7)

 

     (8)

and are the diagonal tensile stress and the

strain of in the reinforced concrete beam,

and are the normal stress and strain,

and are the orthogonal stress and strain

to the corresponding normal stress and strain

and are the shear stress and strain caused by

the shear force along the corresponding normal

and orthogonal stress-strain.

5. Global Damage Mode of the Deep

beam

The failure mode of a deep beam is characterized

by a deep inclined crack that appears to form within

the shear span that does not depend on the flexural

cracks. The cracks are initiated at the bottom of the

beam near the support and propagate towards the

face of the beam, Figure 5, this is cumulated with the

compressive damage by the applied load at the region

of the point load. The global damage forming a tied

arch action. Such behavior has been linked to result

from the stresses sustained by the tension bar

transferred to crack concrete through bond effect at

the ultimate state.

Figure (5): Damage Mode of Deep Beam

Figure 6 shows the principal strain of the damage

mode of the beam when subjected to static load (left)

and fixed-point pulsating load (right) at the midpoint.

Concrete crack was initiated at an applied static load

of about 350 kN at the bottom face of the beam

close to the supports, while crack initiation for the

fixed pulsating loads was around 195 kN equivalent

to about 55 percent of the static load. Propagation of

the cracks flows diagonally across the beam up to the

compression zone forming a bottle-shape. Flexural

crack was formed when the fixed pulsating load was

applied as shown at the underside of the beam, this is

due to the stepwise application of the pulsating load.

Tension bar rupture leads to the rupture of the

reinforcement at the compression zone in Figure 6

(right). Due to prolong loading, cracks of concrete

were formed across all the stirrups and fracture

occurred at least in the first two stirrups close to the

supports. Rupture of stirrup close to the right

support

occurred at around a hundred thousand

(100,000) cycles under the fixed pulsating loading

accounting for a low strain less than 2000μ and total

failure of the beam occurred at over 500,000 cycles.

Figure 7 shows the analytical response of the deep

beam under the static and fixed-point pulsating load

with the maximum capacity of 3360 kN and 3390 kN

respectively which shows the same agreement. This

increase to about 60% of the ultimate force, the

curves exhibit an initial elastic relationship, with

deviation from linearity at around 300 kN and 150

kN for the static and pulsating forces that correspond

to the crack initiation phase. Crack propagation is

symptomatic of increasing loading until the

nonlinearity causes the variation in failure. Quasi-

brittle failure of the concrete in both cases of the

applied load is formed. Strain softening increases in

the fixed pulsating case with larger deflection

compared with the static case.

NJES 23 (3)306-312 , 2020

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311

Figure (6): The principal strain of the Damage Mode of the Deep beam (a) Static point load (left) (b) Fixed-point

Pulsating load (right).

Figure (7): Load-Deflection Response of the beam Figure (8): stress-strain comparison of the static and

the under pulsating loading Static and Fixed Pulsating Load.

The softening properties of the stress-strain

relation adopted in the constitutive model are mesh

dependent which is defined by the finite element of

the beam model; thus, similar meshes are used and

are adjusted to eliminate the inconsistency in the

fracture energy. The strain-softening coefficient in

the plain concrete is considerably higher than the

reinforced concrete element so that damage is

delimitated by the total global failure of the material

(see Table 2).

Figure 8 shows the stress-strain comparison of

the static and the pulsating loads. A localized crack of

the tensile element pickup as shown by a white

rectangle was analyzed as discussed in section 4.1.

there is an agreement between the static and the

pulsating loading up to the proportional limit before

deviation at stress over 250 MPa corresponding to

strain 1600μ. There is an increase in strain at constant

stress between the two curves after their

corresponding yield strain before the failure of the

static load. At the constant stress, the fixed pulsating

load undergoes unloading and further reloading

forming a loop around 7000μ strain which leads to

loss of energy. Total failure formed by the curves is

elasto-plastic fracturing.

6. Conclusion

Behavior of deep beam subjected to static and

fixed pulsating load has been numerically analyzed

using the strut and tie approach, the following

conclusions are therefore reached.

1. The strut and tie model is efficient in calculating

the various stresses generated in the beam, and

hence reasonable estimate of the load carrying

capacity of the analyzed reinforced concrete deep

beam.

2. The 3-dimensional FE analysis of the normal

strength concrete analyzed yielded an accurate

anticipated response as deep beams are

susceptible to shear damage. The shear failure

occurred near the support and propagated up to

the compression zone. Concrete crushing was

observed at the loaded point in case of the

applied fixed pulsating load, this occurred

immediately before the shear crack grows towards

the compression zone at about 500 thousand

cycles.

3. The level of damage is higher under fixed

pulsating loading when compared with static

loading even at lower stress because of the

stepwise loading of the beam, and as the concrete

stress increases, the crack strength also increases.

7. References

[1] Lateef, H.T., Ibrahim, A.Q., "Human Error

Uncertainties for Structural Detailing in Reinforced

Concrete Buildings" . Proceeding of School of

0

500

1000

1500

2000

2500

3000

3500

4000

0 0.5 1 1.5 2

Force (kN)

Deflection (mm)

Static Force

Pulsating Force

0

50

100

150

200

250

300

350

02000 4000 6000 8000 10000

Shear Stress (MPa)

Shear Strain (µ)

Static stress

Pulsating stress

NJES 23 (3)306-312 , 2020

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ResearchGate has not been able to resolve any citations for this publication.

This paper studied reinforcing struts and ties in deep beams based on the Strut-and-Tie Model (STM) of ACI 318M-14. The study contained testing 9 simply supported specimens, divided into 3 groups. The difference between the groups was the loading type which was 2-concentrated forces, 1-concentrated force and uniformly distributed load. Each group contained three specimens; the first specimens in each group were conventional deep beams as references which had a length of 1400 mm, a height of 400 mm and a width of 150 mm. The second specimens were the same as references in dimensions, but with removing shoulders. In addition, only the paths of struts & ties of STM were reinforced in the second specimens as compression and tension members, respectively. The third specimens were the frames that took their dimensions from STM of ACI 318M-14. From the experimental work, it is found that the proposed frames were good alternatives for the references despite the small loss in ultimate capacity. However, these proposed frames already carried loads greater than the factored design loads of STM. In comparison with the references, these proposed frames provided 41-51% reduction in weight, 4-27% reduction in cost besides providing front side area about 46-55%.

The Strut-and-Tie Model, STM, has been widely applied for the design of non-flexural and deep members in reinforced concrete structures. In this paper, strut-and-tie models for selected (shallow and deep) beams with openings, have been suggested based on the available experimental results of; crack patterns, modes of failure, and internal stresses trajectors obtained from elastic finite element analysis. The proposed STM approach is, then, applied to one group of simple shallow beams and one group of simple deep beams tested experimentally. In addition, a three-dimensional nonlinear finite element analysis using ANSYS 12.0 computer program has been employed for two selected (shallow and deep) beams which were analyzed using the STM method. Some of the important factors affecting the behavior of reinforced concrete beams (named: concrete compressive and tensile strength, span to depth ratio, shear span to depth ratio, physical and mechanical properties of horizontal, vertical web reinforcement and main steel, loading position, opening dimensions and location) are investigated throughout a parametric study with the aid of the nonlinear finite element analysis. With such analysis, results of cracking patterns, deflections, failure mode and strain and stress distributions, that cannot be determined using the strut-and-tie model, are obtained. A comparison of the finite element results with test results and STM results has been carried out.

  • Michael P. Collins
  • Denis Mitchell Denis Mitchell

The 1984 Canadian Concrete Code contains new shear design provisions that are believed to be more rational and more general than the shear regulations of the 1983 ACI Building Code. This paper summarizes the new shear design procedures and illustrates their use by means of design examples.

  • Tae-Hoon Kim Tae-Hoon Kim
  • Ju-Hyun Cheon
  • H.M. Shin

The purpose of this study is to evaluate the behavior and strength of prestressed concrete deep beams using nonlinear analysis. By using a sophisticated nonlinear finite element analysis program, the accuracy and objectivity of the assessment process can be enhanced. A computer program, the RCAHEST (Reinforced Concrete Analysis in Higher Evaluation System Technology), was used for the analysis of reinforced concrete structures. Tensile, compressive and shear models of cracked concrete and models of reinforcing and prestressing steel were used to account for the material nonlinearity of prestressed concrete. The smeared crack approach was incorporated. A bonded or unbonded prestressing bar element is used based on the finite element method, which can represent the interaction between the prestressing bars and concrete of a prestressed concrete member. The proposed numerical method for the evaluation of behavior and strength of prestressed concrete deep beams is verified by comparing its results with reliable experimental results.

  • J. Schlaich

So-called 'details' are as important for a structure's behaviour and safety as the standard problems of design which are covered in the Codes. A unified design concept which covers also the details consistently for all types of concrete structure is described in this paper. It is based on strut-and-tie models, including the truss model for beams as a special case. After the principles of the method and the modelling process are explained, simplified rules are proposed for dimensioning all the individual members of the model and their nodes. Some examples show the application of the method and demonstrate, also, its use for the improvement of the conceptual design of details.

Reliability Appraisal of Nominal Eccentricity of Short Reinforced Concrete Column Designed to BS 8110 and Eurocode (EN:2)-Ultimate Limit State on Fatigue

  • Q I Ajibola

Ajibola, Q. I. (2019). "Reliability Appraisal of Nominal Eccentricity of Short Reinforced Concrete Column Designed to BS 8110 and Eurocode (EN:2)-Ultimate Limit State on Fatigue". Reliability Engineering and Resilience. https://doi.org/10.22115/rer.2019.197264.1013.

Behaviour of reinforced concrete deep beams in previous studies

  • K S Abdul-Razzaq
  • A M Jalil
  • S F Jebur

Abdul-Razzaq, K. S., Jalil, A. M. & Jebur, S. F. Behaviour of reinforced concrete deep beams in previous studies. IOP Conf. Ser.: Mater. Sci. Eng. 518, 022065 (2019).