Theory and design of Structures

Mono strand system

Bonded slab

Unbonded slab

PRESTRESSED CONCRETE PART 3 Bonded PT - Stress transfer is through bond (require cement grouting) - strain compatibility between strand and concrete Very common in Australia, Middle East, and Europe Unbonded PT - stress transfer is through anchorage - no grouting required - no strain compatibility - strand and concrete moves independently Very common in North America Mono strand system - strands in tendon are stressed separately usually 1, 3 or 5 strands per tendon Used in slabs or beams Multi strand system - strands in a tendon are stressed at the same time (16, 19, as much as 37 strands per tendon) Used in bridges, and in buildings (transfer beams) PT structures are used in - slab on grade (for houses in expansive soil) - Mat foundation (raft) - parking structures - flat plate - bridges (Girder type, cable stay, suspension) - silos - water tanks - walls (currently becoming common for shear walls in seismic areas)

Prestressed Concrete part 2 Benefit of prestressing system some benefits of using a Prestress system over RC are 1. Longer floor spans and cantilevers 2. As much as 70% saving in reinforcement 3. Thinner slab thickness (as much as 1/3 less than that of RC) 4. High architectural freedom (freedom in column layout, need not to be aligned, can deviate as much as you want to) 5. Removal of form-work in 4 to 5 days after casting 6. bigger floor to floor clear span (this will decrease the cost of facade) 7. very less deflection and crack 8. complete flat plate (no beam required) 9. Decrease structural weight which interns decrease column, wall, and foundation sizes. 10. Decrease the amount of reinforcement required in column & shear walls due to earthquake load, as the seismic weight of the whole structure is decreased. The most common form of prestressing is the post tensioning system. This system can further be classified in to two groups based on type of strand used, and method of stressing Based on type of strand used - Bonded - unbonded Based on method of stressing - mono strand system - multi strand system

PRE-STRESSED CONCRETE PART 1 Pre-stressing is a way of counteracting the effect of external loads on structure by imposing a state of stresses contrary to the load effects Pre-stressing has two parts - Pre-Tensioning Usually straight or harped profiles, mostly used in the precast industry some example would be hollow core slabs, precast beams, Girder beams - Post Tensioning Any type of profile, can be precast or cast in place The only basic difference between pre-tensioning and post tensioning is the state of the structural element while the stressing activities are done. In pretension the stressing is applied prior to concreting while in post tensioning the stressing is applied after the concrete is poured and hardened Further, Post Tensioning is divided in to bonded and unbonded systems; we will get in to this in later days. Pre-stressing a concrete element has two effect axial and transversal based on the tendon profile and anchorage selected. We can have one of the three, Axial (pure pre compression), Eccentrical axial effect, or axial/transversal effect. The later effect is difficult to utilize in pre-tension system

The following discussion about PT are forwarded from a dear friend of mine upon his consent.

It is my strongest opinion that our municipalities should stop the demand for DCM design from designers. This opinion is developed from the view that: 1. DCM design is very complicated and involving for us to follow. I doubt if most designers understood the rules of the code by now. 2. Well designed DCL structure is better than ill designed DCM Structure. we expect the structure to dissipate energy through ductile failures and decrease the amount of force it experiences, a very involving detailing should follow to make this assumption reality. Meaning Structures designed by using EBCS-1995 are going to perform better than those designed by ES EN 1998:2015( one of the reasons for moving to this new code is seismic related issues which are still at large due to wrong implementation of the code...nomal to our country, isn't it?) 3.The rules the code stipulates aren't followed thoroughly( If u ask me, I wouldn't do that much manual calculation to insure ductile behavior in the structure...personal reason for most part!) 4.There is a wrong perception that Software's such as ETABS can do capacity design( very funny perception). This is wrong, in fact very few software's can handle Eurocode and those software's aren't common amongst designers. 5. The detailing rules if implemented fully will create chaos for contractors. With our traditional methods and "senfena" it would be time consuming for contractors to excute the work. I strongly believe the code is doing more harm than its intended benefits.

After beams are taken care of columns continue. One must understand capacity design is hierarchical by it's nature. Special framed Structures should follow a strict order of beam first, column next and then foundation at last kind of order. Since we want beams to fail first, columns degmo before foundations( if yielding at the foundation level occurs first then the building will collapse totally). Walls(ductile) are designed apart from frames and different rules are employed to derive the design shear and bending moment.

Let's take beams to discuss how the non-critical region of the beam should be designef for shear. In order to determine the amount of shear force that the non critical region sustains, once must first design the beam for flexure in order to have the longitudinal rebar at hand. The tricky part is here, T beam effect should be considered. Usually when we design sections for gravity load, since more is better, we neglect T beam effect and also the slab reinforcement within the flanges. This approach is totally wrong and leads to failure since over strength leads to unexpected amount of shear in the section. Following rules in ES EN 1998:2015 part of the slab that should act together with the beam should be identified and the reinforcement within this region should also be included in the estimation of the moment resistance of the section. Once the moment resistance of the section is determined, taking the freebody of the non critical region of the beam and assigning the dead plus quasi permanent live load, the shear force is determined. The non-critical region should be designed for this amount of shear force. We can point out a mistake that Structures design for DCM here in addis are exhibiting. The shear reinforcement detail should have five regions. Usually designers tend to use three region shear reinforcement detail which is wrong.

Critical regions are designed and detailed to prevent brittle shear mode of failure and they should favor bending type of failure in order to help the structure maintain service after the seismic event. The critical regions are designed generously for shear. A densely spaced shear reinforcement should be placed within the critical region and the bending reinforcement should be provided to resist the moment from the inertia force in addition to dead load together with the quasi permanent live load. The shear reinforcement within the critical region is driven from simple code provided equations. This equations usually contain the section depth and for most part they are emperical.

With the help of capacity design rules what we seek is ductile response of the overall structure and energy-dissipation within selected regions called "critical regions". Imagine an electric system. Fuse is installed within the system in order to "dissipate" excess or "abnormal" energy that is coming suddenly. This abnormal energy seldom occurs, yet if it does it should be controlled in order to limit the damage that it does on the whole system. And a fuse burns( i don't the technical term) and it would then prevent the system from getting electricity. Analogous to the electric system, Structures also experience "abnormal" amount of inertia force( energy) during seismic event. This energy should be dissipated systematically in order to limit the amount of damage the structure experiences. One way to dissipate this "abnormal" energy is through critical regions selected carefully. Critical region is a region of energy dissipation. It is supposed to undergo plastic deformation( burns as a fuse does) and help maintain other parts of the structure in elastic region.

Capacity design associated with ES EN 1998:2015 is associated with shear failure prevention rules. Since shear failure is very brittle, the provisions of the code associated with detailing for local ductility are all into facilitating ductile bending failures.

What is Capacity design ? Following the saying that a chain is as strong as its weakest link, one can state capacity design as a design philosophy that aims to protect the structure by protecting the weak links(brittle once) through the failure of the stronger( in the sense of more ductile) links. The links can be interpreted as internal actios such as bending, shear and axial load. From the aforementioned internal actions bending mode of failure is more the ductile than the others, making it the ductile(or stronger link).

One way to understand the scenario is trying to understand how the moment at the column slab junction is trying to induce more shear. From eccentric footing analysis we all know that the stress that the column load induces on the footing varies linearly and is different at the four corners. Using the same analogy, one can see that the same situations occurs on column slab junction and the picture depicts it clearly. Is there any way to prevent brittle shear failure for flat slabs located in seismic zone? Of course there is! Since the drift the system experiences is very huge when flat slab is used, it is advisable to use lateral load resisting systems adequately. Once the drift is limited much of the headache is resolved. The second solution is to design the junction for the expected moments and provide more reliable shear reinforcement such as stud rails. Provision of drop panels or column heads will also play a vital role.

Is there any particular reason to avoid flat slabs in seismic prone zones? One must understand what to expect from Structures in seismic zone in order to judge if the use of flat slab in seismic zones is good or bad. What do we avoid when seismic loads are exerted on Structures? Simple brittle shear failure! Do we design our structures not to fail during seismic events? Nope, we design them to fail in a more ductile manner...mark my words...in a more ductile manner. Shear failure by its nature is brittle. For most part it's because shear by itself isn't a problem, but the tension that shear induces in a plane different from its application is the headache. So why do we bother with shear, flat slab and seismic loads? Usually during seismic event a huge amount of moment is expected to be transferred at the column slab connection. And one must understand a large amount of moment at the column slab junction simply means reduced shear capacity of the slab.