Have you ever seen hidden beam in building? Most probably not, right? The reason is – in completely constructed building, it is invisible in nature. The word hidden itself indicates that it hides somewhere. And that somewhere is slab. We can’t see well-defined rectangular, not ever other because it is casted along with the slab and it is embedded inside the slab. Then the question may arise how a person know there is hidden beam? During reinforcement arrangement, there are stirrups and longitudinal bar just as normal beam in slab portion. As we know, stirrups are not provided in slab of building.
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Introduction of Hidden beam
Hidden beam is the reinforced concrete beam, also called concealed beam provided within slab. The depth of the hidden beam is the same as slab depth but the depth of slab should be greater than 125 mm. Concealed beams are popular and form as the essential part of modern reinforced concrete framed structures. The hidden beam is the virtual beam that was originated by someone to make the use of the beam free head room.
Strict and rigid architectural consideration led to provision of hidden beams. In this manner, the load which might be brick wall will be dealt with and height of floor will not be compromised.
- Concept of concealed beam originated from flat slab concept
- Hidden beam also means to spread the concentrated load of wall on the slab area.
Design of hidden beam
The design of hidden beam is same as the conventional beam design but its depth is restricted and should not be greater slab thickness. The thickness should be 125 mm or more. So it may be requires to increase reinforcement ratio and width of the beam to overcome this restriction to the certain degree.
It should be keep in mind that hidden beam should always be simply supported with anchorage on either ends to beams or columns. May be it is continuous for more than one span but it should also be treated as secondary beam which rest on primary beam.
Purpose of concealed beam
- It is used to help disperse load imposed on the slab ( for example; weight of brickwork masonry wall)
- It allows to use greater span of the slab
- Hidden beam area provided exclusively for the architectural aesthetic purposes in the building interior.
- To create tying effect
Why do we need hidden beam?
When there is restriction to keep normal beam due to architectural consideration then hidden beam become the good option. It create the smooth surface in ceiling which is good aesthetically. Further if we did not want normal beam but have to erect partition wall on slab, then hidden beam helps to distribute the load of partition wall.
Once someone ask, can we build brick-wall directly on slab? Yes we can if we have consider the probabilistic load of brick-wall during design. But it may drastically increase the depth of whole slab with increase in the self-weight which is not suitable for making earthquake resisting building. Rather than increasing the thickness of slab, it is advisable to use hidden beam. It becomes economical, decrease self-weight and leave no impression after casting., ie. it is invisible under slab.
Structurally it creates a spanning problem as spans for the structural support are at the right angle to each other. This means one slab structurally resets over the other. It gives support to slab as the normal beam.
Where it is used in building?
Hidden beam are generally inserted within the suspended slabs where slab thickness is considerable. It is provided between living and dining spaces right angles to each other, this give a neat and level ceiling surface that is good for the look. Hidden beam between balcony and room is very common to facilitate easy inclusion of balcony into the room space later.
- If the cantilever beam is with a “hidden beam” it can’t acts as the beam and will only distribute the load over the slab area and it can’t be considered as simple supported.
- Mostly used in high rise multistory residential as well as commercial building where false ceiling is provided.
Advantages of Hidden Beams
Hidden beam are designed structural element because of several advantages which includes;
- It saves the floor height clearance.
- It saves the cost of framework, labor and materials.
- It helps to decrease the thickness of slab which decrease the seismic risk.
- It creates the acceptable aesthetic appearance which permits for efficient interior space partitioning.
- Concealed beam clears the way for horizontal electro-mechanical duct-work.
- More applicable in commercial building.
Disadvantage of hidden beam
The disadvantage of this beam is that it has considerably low ductility due to height reinforcement ratio which is provided to compensate for the depth restriction especially at column connection.
Types of slab in which hidden beam are introduced
Hidden beam are widely used in three types which includes
- Waffle slab
- Ribbed slab
- Solid slab
Waffle slab:
Waffle slab is also called grid slab, comes from the grid type pattern. It is a two-way joist reinforced concrete slab which is made of concrete reinforcing ribs running in two direction on its underside. It is used where large span are necessary like in a conference hall, auditorium, cinema hall, to avoid many column interfering with the space. The waffle slab can be constructed for the span up to 16 m. The width and spacing of ribs in waffle slab is recommended as 100-200mm and 600-1500mm respectively.
Ribbed slab:
Ribbed slab is a one way joist reinforced concrete floor slab which is made of concrete reinforcing ribs in only on direction on its underside. The ribbed slab behaves as series of T-beam casted in which slab behaves as the flanged portion. The ribs may be tapered in cross-section in its lower part. The spacing between the ribs should be in general 600 – 1000 mm.
Solid slab:
It is the most common types of slab which is used in residential and industrial construction. It has constant cross-section throughout the span. Solid slab are fully customizable concrete slabs which can be constructed for any width, length and thickness.
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