A Changed Paradigm
Thursday, September 23, 2010
A Changed Paradigm
Sunday, July 11, 2010
Monday, June 14, 2010
At POSTEN Engineering Systems, Dennis Reilly developed procedures whereby the Post-tensioning Software, POSTEN Multistory, automatically determines the thinnest section of concrete & the least amount of steel to create the most efficient design possible, completes the design and then automatically produces documentation required for LEED certification.
In Sustainable Concrete Design, the conversation usually starts with Fly Ash or Building Durability and then goes quiet.
In Sustainable Steel Design, the conversation usually starts with the use of Recycled Steel and then, again, goes quiet.
The use of Fly Ash and Recycled Steel is not new, and with the increasing costs of Cement and Steel, their use will expand whether or not Sustainable Design is important or not. In many cases, Fly Ash improves the performance of the concrete from installation to long term durability. With the exception of Specifying the use of Recycled Steel, the use and acquisition of Recycled Steel is mostly outside the control of the Design Structural Engineer. In any regard, Dennis believes that using materials that you would use anyway is not truly Sustainable Design.
In Structural Engineering, Dennis believes that Sustainable Design demands more - Efficient Design. Even though Post-tensioned Structures use significantly less steel & concrete than Conventionally Reinforced Concrete structures, they still tend to be extremely in-efficiently designed, since they are often designed using inefficient & time consuming Trial and Error Methods.
Dennis demonstrated Automated procedures that could save an inch of concrete thickness and 9% in steel in a typical post-tensioned slab, while improving the performance of the structure & documenting the savings for LEED.
Dennis then presented the concept of Post-tensioned Moment Frame Design (only capable on POSTEN Multistory), which further Expands Sustainable Design by removing the need for shear walls (opening up the building).
Dennis believes that , just as Energy Efficient Radiant Heat also provides the most comfortable heating system, Efficient Sustainable Post-tension design should and can expand the volume of the building or reduce the building height, reduce the building weight and enable the Architect’s creativity, while also REDUCING the cost of construction.
Tuesday, March 23, 2010
Tuesday, March 16, 2010
Who should design - the Architect or the Engineer?
Saturday, March 13, 2010
Wednesday, March 10, 2010
The popular trend in Structural Engineering these days (especially with the BIM’s introduction into the profession) is to use Finite Element Analysis software to design concrete structures, whether conventionally reinforced or post-tensioned.
Monday, March 8, 2010
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.
Friday, March 5, 2010
In a typical 150 ft. x 300 ft. Post-tensioned Concrete Slab, if you could reduce the thickness of the slab by 1”, you would reduce the CO2 pollution produced by the manufacture of cement by the same amount of CO2 produced by 4 automobiles in one year.
Imagine, if at the same time, you could reduce the amount of steel used in that thinner slab, Saving Resources and Energy Use - All the way around.
Imagine further that since that slab is thinner, how much more efficient the design is. Seismic and Wind forces reduce with a lower building height. Or the Architect can maximize the use of the building within the height limitations, set by zoning requirements.
Imagine that the building costs less to build; and
Imagine that Structural Engineering Design fees are also lower.
The Usual & Wrong Way:
For Structural Engineers to design Green, normally means Sharpening the Pencil, using newer more time consuming design methods, using very expensive software packages and, in some cases, new structural systems with limited or no performance background.
Design Services tend to be Costly,
Construction Costs may be high,
Testing and Inspections may be extensive.
With new untested systems Construction Liability may go up.
Adequate Documentation for LEED may be challenging.
Sustainable Design produces more efficient buildings and does not have to cost more.
The POSTEN Multistory Way
Taking a Fresh Look at LEED & Believing that Sustainable Design can be done Economically.
We developed Innovative Design Procedures for Post-tensioned Concrete Design that:
Reduce Pollution (CO2 gases), Use of Resources & Energy Use from Construction;
Increase the Efficiency & Use-ability of the Building;
Enable the Architect’s Creativity;
Reduce the Cost of Construction; & Save on Design Fees.
For LEED projects, Documentation is Automatic.
We believe there are also additional Savings to be found in the upcoming Cap & Trade Legislation.
How Do We Do It?
To establish a base line, we produce a quick Conventionally Reinforced Concrete design using our program CONCRET.
Then, we produce a Post-tensioned Concrete Design with POSTEN, which is Efficient (as opposed to our competitors’ merely code compliant designs). Utilizing POSTEN’s proprietary “Slab Optimization” algorithms this run ALSO makes sure that the thinnest section of concrete slab possible is designed.
Then using POSTEN’s proprietary “Stress Balancing” algorithms, we then AUTOMATICALLY produce the Most Efficient Design Possible. This output also Automatically prints out how much steel is saved in the process.
At each stage, POSTEN Multistory produces Batched Tendon & Rebar Schedules, documenting the amount of steel used (i.e. saved) along the way.
All of this is done (including the documentation for LEED) in a FRACTION of the time that our competitors produce an inefficient, simply code compliant design.