Can we use Force Envelope for Design?

Generally, students learn about load cases, load combinations and envelopes etc. when they learn software like ETABS or STAAD Pro after their BE in civil engineering or Masters in Structural Engineering. This is mainly since most of the BE civil engineering curriculum revolves around gravity load cases and design is limited to gravity loads. Many civil engineering students are even confused on the need of load combinations. In this blog, I will try to quickly mention what is Load combination, why we need it and what is a force envelope and why we need a force envelope etc.

Please read the blog fully for completely understanding the subject and context. After that please see the video below for a better understanding the concept of force envelope and where it can be used and where it should not be used. Your understanding on the subject load combination and force envelope will be complete if you read the blog and then see the video. I suggest you do that.



What are loads?


There is no need for me to explain this point. Load is an action. The self-weight of the members, walls, tiles, finishes all forms a part of the dead load. Peoples weight etc. forms the live load and then you have wind load and seismic forces acting on the building. My idea is not to explain these in detail here. Am sure most of you know it. If you have any question on this, please ask in the forum by logging in to your civilera account.


Why should we apply loads separately in software like etabs and staad?


This is a question I recently received from a student. Why can’t we apply all loads in a single case and then avoid having combinations? At least for dead load and live load why can’t we do it? Some of you may think this as well. There are 2 reasons that we don’t apply even dead and live load in a single case. If you look at British codes and many other codes, the partial safety factors for dead and live load are different. If we apply both loads together then we cannot have 2 different factors. As per IS codes both factors are 1.5 but as per BS codes it is 1.4 and 1.6 respectively. This difference can be achieved only by separating load application.

Even the seismic mass which is an inertial mass is derived from the full dead load and a part of the live load and hence separating them is needed for achieving the correct seismic mass.

Similarly, the combination involving wind and seismic also has different safety factors and it is required to separately apply these load as different cases.


Why do we need load combinations?


We all know that the loads act simultaneously and we need to capture the critical loads coming together and hence we need to add them. This is achieved by load combinations. You can also do this manually or using excel sheets. However, this will be a laborious method. As an example, if we want to find the critical moment in a beam in a particular location due to dead and live load, we will have to find the moment due to dead load and moment due to live load and then add it. Like this we have a minimum of 25 ultimate combinations and hence we will have to do it 25 times for a single location of a beam.

Combination option in a software, is nothing but a means to achieve this summation with in the software. This avoids the laborious work that we have to do.


What is a Force envelope?


If we have combinations then why do we need an envelope? As I said we will have 25 to 26 ultimate load combinations and now we have to design a section of a member, say a beam for the maximum moment at a section. You need to manually inspect to know the maximum value. How can this be avoided? How about drawing a moment diagram for combination 1 on a sheet and then use a tracing sheet and draw the moment diagram for combination 2 and then another tracing sheet and then combination 3 and so on? Then keep all the tracing sheets over each other and make a single diagram? This is exactly what an envelope is. A force envelope is a single force diagram that super imposes all load combinations and make it visible for you at a single place. This simplifies your understanding and result interpretation. I suggest you see the video after you read till the end. This force that we superimpose could be moment or shear force or any force. If you do it for moments,you call it moment envelope.


Can we use the force or moment envelope for the design of members?


The answer is yes and no. Yes, in some cases and no in some cases. I have seen engineers blindly taking envelope and designing. If you ask, you will realize that they are following their seniors who are again following their seniors. One should be careful to use envelopes in design. One good thing is that the safety of the structure will not be affected as using envelopes in wrong situations as explained below, generally over estimates the forces and hence we get more sizes for members or more steel. However, this cannot be a reason to misuse the envelopes. Let us have a better understanding of this by discussing a few additional questions as below.


Can we use envelope for design of beams?


Generally, yes but not when you are considering torsion in your beams. Let us first discuss a situation when there is no torsion or when torsion is released. (If you want to know when we can release torsion and when we cannot release torsion, read this blog here.)

When there is no torsion in beam, generally you have to design for moments and shears. Let us assume that combination 1 is inducing maximum moment in a section and combination 10 is inducing maximum shear. If we take envelopes for both then your design moment Mu and design shear Vu is captured from 2 different combinations. Design for these 2 forces is independent of each other. Even though non ductile design for shear is dependent on the amount of steel in the beam, the design won’t get affected because the beam is designed for the critical moment and that required steel is being provided. So, the real additional shear capacity due to long bars are actually present in the beam. So, you can use it for the design of shear which is from a different combination, 10 in this case. So, the design will not get affected. However, if you have torsion in the beam, the case is different. The maximum torsion can be from a different 3rd load combination say combination 5. The design for torsion is all about enhancing moment and shear by an empirical formula in IS 456 and this will result in capturing maximum moment from load combination 1. In fact, we should add moment from combination 5 and torsion from combination 5 and compare with that of combination 1. If you use envelope, you will be adding the moment from combination 1 and torsion from 5 which will result in over design.