Hand Calculations in Structural Engineering. An old fashioned activity or on the contrary a task more necessary than ever.

The employ of structural software is without a doubt an standard in every engineering consultancy office since many years ago. Nobody including myself, could not imagine to face and to solve any structural project without using any commercial, academic or companies in house programs. Structural analysis, getting forces from model of thousands of DOF, providing necessary reinforcement areas in RC critical section or verifying that a steel section ultimate bending moment is bigger than design moments, are undoubtedly repetitive activities that could be performed by a computer faster and in a safer way than any structural engineer.

But Structural software is just a powerful tool that should be brandished by expertise structural engineers or at least supervised by a third one in case of rookies. As long as structural software capabilities are going to be increased significantly year by year, revision after revision, Structural engineering knowledge for all professionals dedicated to this activity shall be increased. One respectable Spanish architect told us that structural analysis and calculations were only valid to confirm dimensions or solutions that all we had in our minds. But I am afraid the sentence can not be applicable to no-linear or dynamics analysis, in which output results are in many times unexpected. As far programming routines will be adopted by many software companies, no-linear analysis will be spread worldwide, converting topics like material behavior time dependency, creep, residuals stress, second order effects or cracking phenomena inertia variation in the same analysis, as a regular basis. Our dear elastic and first order structural analysis will be part of the past, as Hardy Cross Method or Cremona graphical analysis are now. We have the risk to become the hammer in hands of black box software or dedicated software companies, which for sure they will hoard the knowledge and codes, like monasteries in the middle age.

For all those previous reasons, it is more necessary than ever to be able to perform hand calculations, parallel verification or partial checking to compare from the a single step to a whole global structural modal response. Life is not always easy and perfect, and software like many other human creations is not different. My suggestion before initiate any structural model in the computer, is to follow the following steps:

1) Take your notebook, your mechanical pencil and a rubber.

2) Start to sketch structure geometry, identifying main and secondary structural systems. Think about not only obvious vertical load pathway to foundation, take into account how to deal with horizontal forces.

3) Minute all possible acting loads, from gravity to thermal. Do not forget or underestimate loads like earth pressure load, wind. The implications for those kind of loads in big structures are very significant and normally will govern the problem.

4) Do not loose from the very beginning solving structural details and connections. It is better to think first in the global structure behavior (horizontal load pathway to foundation) and after that zoom in into details. I know, Structural detailing is the most exciting part of the job, but the giant must be designed as a whole and not by parts.

5) If the structure is complicated generate several models by software. Evaluate pros and cons, and choose the best option in terms of deflection, regularity or to ease construction.

6) Look for technical references, papers or books to get self confidence during the design phase. Do not be shy or proud. Ask for help, assistance or recommendations for those who have faced similar problems before.

7) Simple numbers shall background any huge problem in structural engineering. Computers allow us to get earlier at home but they do not dictate our labor (why is not running?, where is the mistake).

My pleasure to write down this little contribution, and I hope it will be useful for the readers.

Sergio Rodríguez Morales, Madrid, Spain. October 2021.

ssarquitec@yahoo.es