The BCB has delivered valuable results when it was used to simulate the
destruction of the Pyne Gould Corporation building in Christchurch and
the collapse of the apartment block in Chennai. Both cases have been
described in this blog earlier. Now, a third collapse simulation comes
along as another validation case, it further emboldens us to believe
that the BCB delivers noteworthy results. Kostack studio used the BCB to
simulate the collapse of the World Trade Center 7 that occurred after
the terror attack in New York in 2001. This collapse is still furiously
disputed to prove or disprove conspiracy theories that claim that the
collapse happened by controlled demolition. Kostack studio procured the
detailed construction drawings and built an accurate model
with all relevant structural members in Blender. For the initiating
collapse event the findings of the NIST (National Institute of Standards
and Technology) investigation report were used. According to this report
the failure occurred due to the intense fires that were triggered by the
debris from the other two towers. The sprinkler system in WTC7 did not
work properly. The fire caused the thermal expansion of steel beams that
caused a girder to slide and loose its bearing which in turn caused a
vital pillar (column number 79) to buckle. In the Blender model column
79 was removed.
The load bearing structure of the WTC7 was made of steel. The steel
members were bonded with screws. To replicate these connections the
strength formulas in the BCB perform a rough approximation of the ratio
of the total screw area section and member contact area.
The Blender simulation replicates a peculiarity that is not evident when
watching the actual video footage from the collapse. The building’s
structure was a typical tube-frame design, the facade columns and beams
formed a rigid frame that delivered a strong structural membrane along
the exterior of the building. After the implied column failure the
structure collapsed from within and left the hollow membrane staying for
a while before the latter was caving in on itself as well.
This simulation is a sober re-creation of the alleged damages caused due
to the fires that were stated in the NIST report. It is not meant to
comment on any of the claims regarding the conspiracy theories.
WTC7 technical drawings:
In order to supply rescue personnel with training material a data base with pre-run collapse simulations was created. This building library consists of four building types that are archetypical representations of a number of commonly built structures:
1. Vitruv Building #2: a low apartment building
2. Vitruv Building #3: a high rise building with a square shaped floor plan and a center core
3. Vitruv Building #9: a high rise building with a rectangular floor plan and a center core
4. Vitruv Building #11: a historic, double wing brick building
Those four buildings have been submitted to various explosion and earthquake scenarios. Damages from detonations were defined and earthquakes with four ascending intensities from low to devastating were simulated.
This building library is expected to be extended in the future to not only serve training exercises but also allow more and more accurate predictions where hollow spaces will be formed in which victims can survive.
In the beginning the videos show a new useful BCB functionality: A mesh of the concrete rebar is generated following the definitions in the BCB´s Formula Assistant namely rebar position amount and sizes. This rebar mesh is only for diagnostics to detect obvious mistakes in the element settings and is not linked to the simulation routine.
We picked up our initial approaches (blog post from 20. Mai 2015) to visualize cavities and applied a modified version on the collapse debris mesh. We could prove that cavity can be visualized fast and directly in Blender largely with Blender´s native inbuilt modifiers. The result can be seen in the last sequence of the videos.
We are happy that our Pyne Gould simulation was noticed by the creator of the Bullet Physics library himself. The video is now cross-linked on his personal Google+ profile:
The simulation of the Yu- experiment, for the first time really put our work we have done so far to the test in terms of physical accuracy. We have built the beam with increasing discretization (subdivision) levels. We approximated the weight needed to break the setup by running multiple simulations for each subdivision level. We where very happy to see that the finally established weights of 5100, 5400, 4800, 5000 kg etc where all in a very close range compared to the real loading level of 4700kg in the original real world experiment. (maximal deviation +15%)
We emphasize that at the moment we do not track permanent plastic deformation or crack propagation. What at this stage is interesting to us is to evaluate if we can predict the absolute moment of breaking of a structural assembly. This first comparison with a real world experiment seems to indicate that this is the case.
To validate structural performance of reinforced concrete structures many tests on RC assemblages have been carried out under column removal scenarios. They provide valuable information on structural dynamic performance. One such experiments was executed at the Fraunhofer Institute for High-Speed Dynamics by following the specimen design by Jun Yu and Kang Hai Tan: A ca 6 meter long concrete beam supported by a center concrete pillar was loaded with a constant 47kN in form of two huge concrete block weights . The center pillar was then suddenly removed by explosives.
The research papers by the Fraunhofer Institute can be downloaded from here.
Accurate formulas are now integrated in our script. They allow breaking threshold calculations based on reinforcement information. Amount and layout of the steel in a building element are now taken into account. Input parameters are for example diameter of the irons, amount of rods, distance of stirrups, concrete cover.
In the following weeks we will have a splendid opportunity to test the accuracy of the script. We will cross validate our simulation results with real world experiments.
With the support of the engineer office Schüssler-Plan we have collected a set of formulas that is now enabling us to take into account the steel reinforcement in a building element.
Steel reinforcement can improve particular strength values. For example to improve the performance under shear force steel stirrups are added, or to improve the resistance against compression and tensile forces longitudinal irons are added etc.
These Formulas can now be applied to calculate quite accurately the threshold values we need to be set for our particular constraints. We created an excel table to run the calculations on a test bases.
Next we will implement the formulas directly into the BCB- addon. The script will calculate relative strength values: 160131-formulas-calculation, tensile, shear and bend based on a sample element and then apply those relative values in N/mm2 to all the other elements within the same element group.
We are waiting with excitement to the results of the first validation cases.