Seeing the title, many young and entry level civil & structural engineers and students may think of short columns and may feel, is it not good for the column to be short? Yes we all know that when we compare short column and long column, the long column will need slenderness moments to be considered as per IS 456 and short columns need not be designed for any slenderness. However that’s about long columns and columns that are just short. Too much short columns can create undesirable forces in the structure.
What is Short Column?
As per the IS 456 defenition, a column is short when the ratio of effective length of the column to its dimensions are less than 12. So a column can be short in one direction and long in other direction. The column can also be short in either direction or long in either direction. This all depends on beam depths in either direction. Any ways intention of this blog is not to discuss the slenderness effect.
What is short column effect?
Many young civil engineers get confused between the short columns and short column effect. While a short column is looked up on as a better thing compared to long columns, the short column effect is not exactly just a non-slender column. If the column is too short, there is an undesirable effect and that’s called short column effect. The short columns are stiffer as it is supported more closer i.e., its effective length is less. If 2 columns of same dimension undergo same deflection Δ horizontally, the short one attracts larger seismic forces as it is stiffer. Since the slabs provide diaphragm action to the floor, in a laterally loaded building, the columns are supposed to undergo same deflections and as I mentioned, the short columns would attract more forces and if not properly designed will be subjected to X-Shaped cracks and damages.
I will explain short column effect with a few practical examples.
Building on a slopped ground
When we have buildings on slopped ground, it is more likely that the foundations will be at different levels. This is particularly true when the soil is hard. We cannot always have all the footings at same level by having basements etc. in such cases. This will result in some columns being short and some much longer than the shortest. As you know, when any member is short in length, then it also means that the member is stiffer as the end supports are closer. Mores stiff members do attract more seismic forces. This results in more damages to the short columns.
Mezzanine Floor
Many times, especially in commercial or storage buildings, there will be a mezzanine floor. A mezzanine floor is nothing but an intermediate floor usually around the periphery of the floor. The floor height may be 4m and at may be 2.1 to 2.5m, you may have an additional slab to store or stack some material. This requirement creates a requirement for 2 beams to be closer and hence short columns.
Stair Mid landing beams
Stair mid landing beam is a necessity for any project. Sometimes the flights may be unequal and the landing may be higher than mid-level and that makes the landing beam even closer to the floor beam. This makes the column really short.In this picture, the beam is a cantilever but the beam can be continuous inside connected to the next column too and thereby making 2 columns short. Remember that there will be a floor beam too. If the floor height is 3m, then the column length is only 1.5m. If the flights are unequal then the beam can be at a different level than mid-level, thereby making the column even shorter.
Continuous Lintel
In a framed structure, lintels need not be always continuous. They need to be provided only where there are openings. The floor level beams will tie the columns and that will be good enough for seismic resistance. However, there may be situations when the openings are large and there is no meaning in providing cut lintels only above the openings. This will result in the lintel being continuous and connected through columns. he floors beam of course will be at the regular floor level. This will reduce the effective length of columns and will result in more stiffness. This creates short column effect. Sometimes, in higher seismic zones, if the seismic demand is high, engineers provide beams at sill level, lintel level and then at floor levels. These should be carefully analysed to capture the effects of short columns. What I mean is, it should not be just provided at site without accommodating in the model. Many buildings are not engineered and the site engineers or the contractor will provide these beams as a construction practice with out designing. This should not be done with out analysis and design.
Veranda/corridor
This is an additional case of a situation when the lintel has to be continuous. A typical example is a school building where the classrooms are of higher level and then you also have a veranda or corridor which is at a lower level. This results in the need of a beam to support the veranda slab. This also results in short column. There will be one beam for the roof slab and another for the corridor. The short column will be tucked between two beams. This will be a very short column and is very undesirable. Many times, it will be better to make the beams deeper than providding two beams that close. If ever it is provided, ensure that both the beams are modelled in ETABS or which ever analysis tool you are using.
Large Openings in walls
This situation can result in stiffness variations and hence create a short column effect in some columns or some portion of the column. There can be situations where one frame is having a partial opening and other full opening. The columns in the bay with partial fill of walls will attract more lateral forces. This is an undesirable situation and shall be avoided in the planning stage itself. You can see in the image here that some walls are partial and some are full. Some bays don’t have walls. Say it is fully glazed. The columns that have part walls may behave as short columns.
What is the Solution to Short Column Effect?
There are possibly only two solutions for this.
Avoid the bad effects by proper planning
Accomodate the short column effects by proper modelling,analysis and design
In old or existing buildings, you may have to retrofit to suit.
I have explained it below. Also have provided the IS code provisions.
Avoid by proper planning
One solution is to avoid the above-mentioned structural conditions in your project. However, that is something related to functional planning of the building. A structural engineer may not be on board during architectural scheme development. So, it is important either the Architect knows about this basic structural need or a structural engineer is on board early enough to point this out. Many times, you may not be able to avoid this. It is very common to have the situations that I mentioned in projects. So, avoiding them may not be really possible all the time. If that is the case, accommodate it as explained in the next section.
Accommodate by Design
You can avoid short column effect by accommodating it by proper modelling and designing. What do you mean by proper designing for short column effect? It is nothing but ensuring that you model all the elements in your ETABS model or STAAD model thereby ensuring that the analysis capture the short column effect. Many engineers do avoid modelling of mid landing beams of stairs and lintels and then apply an equivalent load on columns and floor beams. This should not be done because the undesirable short column effect can be captured only if you model everything properly as in site and structural drawings. Once you model, all the forces will get generated in the analysis and as you design for these forces, every undesirable effect will get designed for.
Code IS 13920 Provisions for Short column effects
IS 13920 2016 Clause 5.4 suggests not to integrate columns and lintels. This means to have cut lintels only above windows and doors and not to run through connecting columns. If included, code mandates that you have to include it in the analysis. Same mention about stair landing beams and plinth beams.
The code even mandates to ensure you model the slabs that are framing in to the column. In fact, all slabs are monolithic with columns and you cannot just say slab is supported on the beams. At the junctions all columns, Beams and slabs are meeting and it is all a single piece. Now what does code mean by saying ‘’model the slabs’’? It means you have to model it as shells. By modelling a slab as a shell, you are bringing the slab stiffness in to analysis and any short column effect, will be automatically captured.
Ductile detailing as per IS 13920 shall be adopted for the short columns. The shear reinforcement shall be provided in the end zones of the columns and this special confining steel shall cover the short column and also extend in to the column vertically above and below the short portion by a certain amount.
Existing Buildings
If any of the old buildings has any of the above-mentioned undesirable condition, only way is to retrofit the building to correct the effect. This needs separate study and consulting and varies from building to building and situation to situation. If there are large openings that results in difference of wall heights and hence the stiffness, the openings can be considered to be closed if functionally possible. If not, the only solution is to refurbish it.
Summary
A scheme of a building is not just the positioning of columns and its orientation but also involves eliminating all undesirable effects. A proper involvement of structural consultants at the right time helps to provide inputs to clients and Architects at the right time to make the structural scheme robust. While it is not possible to avoid the short columns all the time, it is possible to model it in analysis and ensure that the undesirable effects are captured.
Many engineers don’t know about this fact. I suggest you like this blog post and also comment your structural questions and then share it too in social media so that it reaches maximum civil engineers, structural engineers, architects and clients. Propagate structures for overall wellness of civil engineering? Its in your hands to propagate our wellbeing.