Shanghai building collapse
On June 27, 2009 in Shanghai City's Minxing District an apartment building collapsed while under construction. Please visit http://zonaeuropa.com/200906c.brief.htm#012 as the original source. I admit that my background is not civil, structural, or geotechnical engineering, but as part of me becoming an engineer I will attempt to explain what I believe happened. I do not wish to sound like I am criticizing the design or construction, I only wish to point out probable causes and learn from this event. So to outline this blog entry, I will go through the following factors: weather, groundwater, soil, rebar, concrete, and the design.
First lets start off with the external condition: Weather. According to Weather Underground that month the city only received 2" of rain and at most 15 mph winds, so I don't believe this is enough to have a major effect.
Next external condition would be the Groundwater. Since the building is fairly close to the local river, I would expect the groundwater to be anywhere from 5' to 20'. This is important because uneven groundwater under the footing affects the buoyancy of the footing which means the size of the footing would vary from one side to the other. If the bearing load on these footings are not properly distributed then the building would lean several inches and possibly causing the collapse.
The next factor is soil which I can only make assumptions. The soil looks like clay. It needs proper soil compaction, otherwise the weight of the building will compress the soil unevenly. Thus causing an unleveled building. If water is introduced to dry clay, the clay will try to expand. Expansion again causes the building to be unleveled. All this in combination with the groundwater and the near by river will reduce the bearing capacity of the soil.
From the photos the Reinforced Steel Bar or "Rebar" was not strong enough to hold the concrete. I see something like #3 or #4 rebar (#10 or #13 in metric) which appeared to have snapped. The purpose of rebar is to add tensile strength to concrete, meaning it will not break as easily if you pull on both ends. If the rebar was strong enough to hold the footing, then you would see the pile caps in the soil unearthed. On another note the rebar does not appear to be ribbed. It almost looks like the rebar is smooth wire. A ribbed rebar allows for more grip with the concrete. I believe smooth rebar pretty much defeats the purpose of having rebar at all.
I hate to also be bias, but my construction experience has told me China doesn't have the best quality control. This leads to my next point: the concrete mix. The water to concrete ratio is important. Too little water the concrete becomes unworkable, but adding too much water and the concrete will be weak. Concrete goes through a chemical reaction process of hydration, it doesn not actually dry as people may think. It needs fairly pure water in the reaction, or else side reactions. So weak concrete can caused by having too much water or using poor quality water.
My final note is being cheap on the design and materials. Money tends to be a common issue in construction where someone tried to go cheap, from either the owner, construction manager, engineer, or supplier. This is possibly the worst thing one can do because it puts people's lives at risk. Someone once told me that when doctors mess up they might have one dead person, but if an engineer messes up they might end up multiple deaths. Unfortunately, one person did perish from this event. Rest in peace.
Well, that's what I have to share. Your comments are welcome.
----
Please see the images at http://zonaeuropa.com/200906c.brief.htm#012 as reference.
First lets start off with the external condition: Weather. According to Weather Underground that month the city only received 2" of rain and at most 15 mph winds, so I don't believe this is enough to have a major effect.
Next external condition would be the Groundwater. Since the building is fairly close to the local river, I would expect the groundwater to be anywhere from 5' to 20'. This is important because uneven groundwater under the footing affects the buoyancy of the footing which means the size of the footing would vary from one side to the other. If the bearing load on these footings are not properly distributed then the building would lean several inches and possibly causing the collapse.
The next factor is soil which I can only make assumptions. The soil looks like clay. It needs proper soil compaction, otherwise the weight of the building will compress the soil unevenly. Thus causing an unleveled building. If water is introduced to dry clay, the clay will try to expand. Expansion again causes the building to be unleveled. All this in combination with the groundwater and the near by river will reduce the bearing capacity of the soil.
From the photos the Reinforced Steel Bar or "Rebar" was not strong enough to hold the concrete. I see something like #3 or #4 rebar (#10 or #13 in metric) which appeared to have snapped. The purpose of rebar is to add tensile strength to concrete, meaning it will not break as easily if you pull on both ends. If the rebar was strong enough to hold the footing, then you would see the pile caps in the soil unearthed. On another note the rebar does not appear to be ribbed. It almost looks like the rebar is smooth wire. A ribbed rebar allows for more grip with the concrete. I believe smooth rebar pretty much defeats the purpose of having rebar at all.
I hate to also be bias, but my construction experience has told me China doesn't have the best quality control. This leads to my next point: the concrete mix. The water to concrete ratio is important. Too little water the concrete becomes unworkable, but adding too much water and the concrete will be weak. Concrete goes through a chemical reaction process of hydration, it doesn not actually dry as people may think. It needs fairly pure water in the reaction, or else side reactions. So weak concrete can caused by having too much water or using poor quality water.
My final note is being cheap on the design and materials. Money tends to be a common issue in construction where someone tried to go cheap, from either the owner, construction manager, engineer, or supplier. This is possibly the worst thing one can do because it puts people's lives at risk. Someone once told me that when doctors mess up they might have one dead person, but if an engineer messes up they might end up multiple deaths. Unfortunately, one person did perish from this event. Rest in peace.
Well, that's what I have to share. Your comments are welcome.
----
Please see the images at http://zonaeuropa.com/200906c.brief.htm#012 as reference.
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posted by David Li @ 11:17 PM
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