Introduction To Deep Beams

Deep beams transfer a significant amount of load to the supports by compression force combing the load and the reaction.

Fig.1. Deep Beam Load Action

Deep beams are beams categorized by the ratio of their effective span (Leff) to the overall depth (H) of the beam as shown in the fig.1 below. For a beam to be categorized as deep beam, its

Effective span/ Overall depth = leff/D <2 As per IS: 456-2000

Clear span/ Overall depth = Ln/D <5 As per ACI Code 

Certain studies define the shear span-effective depth ratio av/d is <5 as shown in the fig.1.

Effective span is generally, the centre to centre distance between the supports that carries the beam as shown in the figure. The value of effective span may vary based on the standard design codes followed by the country.

Short span beams with heavy loads, transfer girders and floor slabs under horizontal loads are examples of deep beams.

Types of Deep Beams

Deep beams can be classified into 

1. Simply supported deep beams 
2. Continuous deep beams
3. Top loading deep beams
4. Side loading deep beams
5. Bottom loading deep beams

Simply supported deep beams those beams with 

leff/D ≤  2 

Continuous deep beams are those beams with

leff/D ≤ 2.5

The representation of top, bottom and side loaded deep are shown below.

Features of Deep Beams

The elementary theory of bending followed in shallow beam design and study cannot be applied for deep beams, even within the linear elastic assumption. Hence, the shear deformation becomes significant compared to pure flexure in the case of shallow beams. 

Also Read: Simple Bending Theory

The stress distribution is linear and straight for shallow beams. But, in the case of deep beams, the stress or the strain across the depth of the beam is no longer straight. It is dependent on the aspect ratio of the deep beam.

The deep beam acts like a vertical plate subjected to loading in its own plane.
The above features of deep beam make it essential to analyze deep beams through 2D stress analysis method so that a realistic stress distribution is obtained to get a solution.
Deep beams undergo failure due to shear deformation unlike shallow beams. 
During the initial action of loads, in continuous deep beams, negative cracks occur at the interior support.fig.2 (a)

Fig.2.

After which, significant cracks form like diagonal shear cracks at about 50% of the ultimate loads and the cracks delineate a truss or tied arch mode of behavior. Fig.2(B).
This later develops and grow to secondary flexural cracks. (fig.3)

Fig.3.

Also Read: What is the Difference Between Beam and Girder

Failure of Deep Beams

Deep beams subjected to a central point load or two symmetrical point load is through the failure of the tied arch that is formed in the beam after diagonal cracking. 

1. The failure of tied arch happens due to flexure failure and shear failure.

2. When the concrete rib of the tied arch fails by crushing at the crown, then flexure failure occurs. Fig.3. At this stage, the beam realize its full flexural capacity and ductility.

3. Shear failure of tied arch happens either by diagonal compression failure or diagonal tension failure. 

Also Read: What Are Simply Supported Beams?