INGENUITY, PART 1: [STRAP FOOTINGS]
Ingenuity was called into question when I faced a challenging task that I usually dont meet.The column to be built was too close to the boundary. Ofcourse my worry was its foundation which was to project into the foundation of boundary wall. I instructed the contractor to do that but later we found out that the boundary wall would fail and believe me, I was not interested in extra costs or neighbour interference. I later found out that the contractor had gone a step ahead to preserve the dimensions of the footing by re-positioning it so that it just ends at boundary wall footing. But he had not changed the column position and I could not have allowed it anyway because of it was locked in grid with other columns. And so that column was now closer to the footing edge and no longer in its middle.
I had not faced such a situation before. I had to delve into first-time principles. Creative engineers have used intuition when faced with new challenges, I thus followed suit:
- One was to internalise the problem
- Two was to internalise the solution.
- Three was to work out the calculations
- Four was to specify the materials to the contractor.
INTERNALISING THE PROBLEM: [MOVIE-1]
In my mind's vision, I saw a load path from the column and into the footing, creating an eccentric effect that led to overturning gestures thus creating weak points. It was like a movie playing in my mind. I had to concentrate my thought energies to keep the movie on. Next, I played and replayed movie clips in the internal vision taking into considerationthe nature of the surroundings close to column and its footing. In the replays, I saw two possible scenarios:
1. a hinge was created at the column-footing intersection and a crack in the floor next to the column ensued as a result. I saw why this had happened.
and/or
2. hogging moments created in the footing next to the column which would come first before any possible failure at the column-footing intersection.
This internalisation took about a minute. I was quite for that period while the contractor was there wondering what I am up to.
And so the weak points had been located. Solutions had to come in next.
INTERNALISING THE SOLUTION: [MOVIE -2]
In my mind's vision, I dared not view the load paths for all the 16 columns. It would get complex and the movie would get distorted. I concentrated on two columns. I thought of a base beam(or reinforced strip footing) which connected the eccentric column to the other column and made it to sit on top of the footings.
Now I replayed movie-1 in my previous internalisation. This time I saw the introduced beam at work, seriously reducing on the overturning effect. The more I made the beam rigid, the more I saw the residue load from the eccentric effect being transfered into the next column footing.
The problem had been solved. The possibility of a plastic hinge at the eccentric column-footing intersection was no more and the hogging moments in the footing was no more and likewise the floor was safe.
Now all the contractor needed was for me to specify the reinforcement and concrete grade for the new base beam. I had to do a bit more calculations for this.
To my surprise when I went into my reference books, in the foundations section, there I saw it; in the book, this kind of arrangement was called strap footings. In otherwords my challenge was not new, many others had faced this same problem and my work had been made even more easier at calculations. Whoa, talk about ingenuity. Engineers are never taught that in school, are they? If is the foundation for design and construction, isnt it?
ciao!
Nathan.
E.Africa(uganda).
MSCE, BSCE.
M_UIPE, M_IABSE.
