Traditionally Multistory Concrete Moment Frame Structures have been conventionally reinforced concrete. Originally this was because of the higher cost of Post-tensioned concrete (as a system) and the relatively low cost of concrete and mild steel. For years most post-tensioned concrete structures were single level concrete podium decks above parking in an apartment complex or in some multistory parking structures. (While there were many multistory post-tensioned concrete structures built in the very early formative years of post-tensioning, this form of construction fell out of favor for many years until the cost of concrete and steel skyrocketed to present levels compared to the post-tension installation cost.) But in both of these podium or parking structure cases, most often in the past, concrete or masonry shear walls were installed to resist horizontal wind or seismic forces. Again, the cost of concrete and mild steel was still low enough to justify a structural system (i.e. conventionally reinforced concrete) that uses a larger amount of steel and concrete.
Now, the cost of steel is very high and the environmental impacts of the manufacture of cement have been demonstrated to be significant.
So, now in the past few years we have seen an explosion of many new multistory post-tensioned concrete buildings. The construction of conventionally reinforced concrete multistory concrete buildings appears to have essentially disappeared.
But, most of these new post-tensioned buildings are still shear wall structures (Not Moment Frame). Why is that? - when the needs of saving materials, reducing the cost of construction and improving the use-ability of the building would indicate Moment Frames?
1. In many cases the design of the post-tensioning is being performed (or controlled) by the tendon installer, not the Structural Engineer or the Architect. The tendon installer has only one agenda and that is the post-tensioned slab. It is what they install and are responsible for. While the engineers that work for the tendon installers tend to be very knowledgeable about post-tensioned slabs, they may not have the appropriate knowledge for the design of the building as a whole. When the tendon installers design the post-tensioned slab, their priorities are not the impacts of the post-tensioned slab on the structure above or below. In their eyes, that is the responsibility of the Structural Engineer of record. In some cases this has led to the failure of the structure through bending of the exterior walls and columns from the lack of coordination between Tendon Installer and the Structural Engineer such that the Structural Engineer was not aware of the secondary moments being applied to the connection between the columns & walls and the floor slabs or the forces being applied to the columns and walls from the shrinkage of the slab. As a result, we have always recommended that the Structural Engineer perform his/her own design of the post-tensioned floor slabs.
This design by the tendon installer leaves only one option for the Structural Engineer of Record & (more importantly the Architect) and that is to design the structure as a Shear Wall Structure. To design the building as a Moment Frame requires that the columns be designed in concert with the design of the floor slabs. It must be a unified design by the Structural Engineer, not something piece mealed. In fact, in our opinion, in ALL situations the time to design the columns is when the slabs are being designed, not after and certainly not before.
In the case of the tendon installer designing the post-tensioned slabs, the Structural Engineer of Record is ultimately responsible for not just the design of the foundation, columns, walls & slab interface (that he/she is being paid for) but the design of the post-tensioned slabs (which he/she is NOT being paid for). If there is a failure in the slab, there will be just as many Attorney’s fingers pointing at the Structural Engineer than the tendon installer.
2. The statement “Shear wall structures is how everyone designs them”. As mentioned above, this is primarily because the tendon installer (who only designs the slab and only cares about the slab, and in many cases uses computer software that is incapable of designing anything but the slab) is putting the Structural Engineer and Architect in a position where a shear wall structure is the only option.
Aside from that, why would you NOT want to design a shear wall structure? The problems created by the existence of shear walls are related to the restrictions on the placement of the shear walls (typically toward the center of the building – avoiding the exterior corners) and the potential for cracks developing in the concrete or masonry wall or post-tensioned slab due to the forces applied to these connections (whether they’re shrinkage related at the exterior of the building or chord forces at the ends of the shear walls). There are many examples of large cracks occurring in the slabs and/or walls from these conditions (that may be compounded when masonry walls are installed in wet conditions).
3. On the flip side, while multistory post-tensioned concrete buildings have been permitted in high wind or moderate seismic regions, until ACI318-05, multistory post-tensioned concrete moment frame structures were not permitted in high seismic regions, like California. ACI318-05 now permits it’s use under certain conditions (i.e. the mild steel is required to essentially resist the full force of the earthquake). As a result, in high seismic regions, the reasons to use post-tensioning in a multistory moment frame structure would be to control deflections. However, in high wind areas battered by seasonal catastrophic wind storms or in some cases ocean wave surges, the survival of the building may depend on the construction NOT including shear walls, but moment frames instead.
In any regard, we believe that in so many cases, a multistory post-tensioned concrete moment frame structure would be the most appropriate design solution.
Here at POSTEN Engineering Systems, we produce POSTEN Multistory, the Only Software in the World that designs Multistory Post-tensioned Concrete Moment Frame Buildings (for Wind or Seismic Forces utilizing first or second order analysis – with P-delta effects).
Partially because of the much higher level of engineering required to perform this type of design in the first place, POSTEN Multistory is the Most Comprehensive, Powerful, Efficient – as well as Easy to Use Post-tensioned Concrete Design Software.
Additionally, POSTEN Multistory is the only Post-tensioned Concrete Design Software that produces Sustainable designs along with the documentation of material savings required in LEED projects.
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