Working Stress D(isaster)

It all started when a multinational client gave us a call and said that we’ve won the bidding and we have to make the necessary preparations for the project and answer some questions that were found in our bid document. One of these question is the structural integrity of the building. While I have ample experience in the design of building it have never occured to me that this will be the biggest professional challenge for me. One that would challenge my professional pride and personally humble me.

What I thought was an easy task became a nightmare for me. First, the plans that were obtained are basically incomplete and the prospective owner can’t give us the complete details because there is no complete details in the first place. I have only obtained the basic plan for about a third of the building and I am expected to make a quick report for that. I was also informed that deadline is tight.

What I’m holding on is the simple fact that structures are basically designed with symmetry. But I have to warn that even though there is symmetry in the design you cannot just assign a particular cross section of a given beam detail to the missing parts and say that’s just about it. After a few days of research, consultations and heavy prayers (I still can’t believe that the biblical verse is not appropriate but the Lord knows better…  more on that in the upcoming blog) the plan of actions were as follows:

1. Determine the time of design and construction. In this case it was determined that the structure was designed during the mid 1980’s therefore I can conclude that it was designed using the Working Stress Design (WSD)method.

2. Make an initial design of the structural member using the method of design from the period starting from the roof members (luckily im only limited to the roof members and the columns) and compare them with the members that you have. This took me a while since I have to review my WSD (Engineers who graduate at the start of the 21st century are not known to design buildings using WSD) The fact that I teach would make it even more necessary to have a working knowledge of this method. It is also advisable that the member analysis should be made using approximate method (Moment Distribution Method and Cantilever Method since this is the most common method of structural analysis during those period)

3. Compare the structural details with the plans in my possesion right now. I was able to notice that what was in my trial design is lesser than what is in the plan. It could only mean that the designers over designed the structure to ensure that the structure can withstand the earthquake generated from the nearby Marikina Fault Line. What makes my situation complex is that the seismic design that I used is alread the Association of Structural Engineers of the Philippines (ASEP) earthquake manual of 2004.  

In this method only 45% of concrete strength is used in flexure while 25% for absolute compression. While in the present method, the Ultimate Strength Design (USD) method up to 85% of the concrete strength is utilized. However before I can say that it is safe, I also have to consider the fact that WSD loads are not factored, thus it is not yet clear why