Seismic Design Philosophy: Stronger not always Better
Summary:
This fourth article in the series focuses on fundamental concepts of earthquake-resistant design of buildings. The article is technical in nature and is written more for professionals in the building industry. It stresses the fact that strength is not the only important factor necessary for good seismic behaviour, and points to the added need for ductility. The seismic design concept of capacity design that ensures that buildings respond safely in earthquakes is also discussed.
Good or Bad Advice?:
A story is told of a senior engineer (SE) and an engineer-in-training (EIT) who were working on the design of a reinforced concrete beam and column multi-storey building. When the SE prepared the bid for the job, he quoted a rather low price and also submitted a very tight schedule for the design. After they won the bid and started work, the SE told the EIT that they had two weeks to complete the design. The EIT was puzzled and asked the SE, how they could meet such a tight deadline. The SE smiled and told the EIT, “Throw in lots of steel! To be a successful engineer, this is a lesson you must learn! You do not need to design everything, just copy from a previous job and make sure there is enough steel to make the building strong.” The face of the EIT lit up, and he thanked the SE for the valuable advice and started working. When the submission day came, the EIT submitted the drawings to the client; the client then remarked, “Thanks, this project has been done on-time and on-budget.” The EIT left with the feeling of a deep sense of accomplishment.
Among many others, an important question that could be asked is whether the advice the SE gave the EIT to add lots of steel to make the building strong was good or not. In other words, does making a building stronger, necessarily make it safer? This article will argue that this is not necessarily true, particularly in the context of earthquake-resistant design.
Strength not Enough:
Earthquake waves impose vertical and horizontal forces on a building. The vertical force is counteracted by the weight of the building, and is generally not damaging. However, this is not always true and needs to be carefully assessed. The horizontal force imposed by an earthquake on a structure is an inertia force that is generated as a result of that structure experiencing acceleration. This is similar to the force a person in a car experiences when the driver makes a sudden stop. The forces imposed by strong winds on a building are also horizontal; however, in contrast with earthquake forces that are internally generated, wind forces are external forces that are imposed on the building.
An important difference between the two types of horizontal loads is the way they are handled by modern seismic design codes for new buildings. Under design wind loads, a structure is expected to remain elastic without incurring any damage. In contrast, under design earthquake loads, a structure is expected to sustain some amount of damage; however, this is not expected to result in a reduction in its strength, leading to eventual collapse. This approach is followed because designing structures to make them strong enough to resist expected earthquake forces without damage is an expensive option. This option is used mainly in the design of critical structures such as nuclear power plants.
When a structure is able to deform continuously beyond its yield point without significant loss of strength, it is referred to as a ductile structure. On the other hand, structures that loose virtually all their strength upon reaching their design maximum load are termed brittle structures, and perform poorly in earthquakes. Ductility is therefore a very important aspect of earthquake-resistant design.
From the above, the goal of earthquake-resistant design is not to prevent damage, but to prevent collapse, leading to loss of life. To achieve this, the designer must ensure that under the design earthquake load, the structure is able remain stable, although damaged. This is achieved by exercising control on the building’s behaviour by pre-determining how it should respond in an earthquake. The approach used to do this is termed capacity design.
Capacity Design:
Earthquake forces are the most unforgiving a structure can experience. It has been said that if you want to assess how rugged and robust a structure is, just put it on a shake table (a shake table is a mechanical device used to simulate earthquakes). That is to say, earthquake forces easily expose all design flaws and/or design inadequacies of a building system. This is in part due to the fact that earthquake forces are generated through out the entire building, and must all find uninhibited paths from their point of action to a point of dissipation. This means that load paths for vertical and lateral loads must be clearly defined in a properly designed building, and must be capable of effectively transferring loads from the point of action to a point of dissipation.
If along the load path, any points of weakness or bottlenecks are encountered, that point gets overstressed, and if not designed to respond in a stable ductile manner, fails in a brittle way. This then leads to the other parts of the structure getting overstressed and also failing in a brittle manner, with the final eventual collapse of the entire structure. This form of progressive failure is responsible for the “pancake” manner in which many buildings in Haiti failed. Capacity design ensures that this does not happen, and it does so by ensuring that a structure has load paths that are CAPABLE.
Considering a multi-storey beam and column building (i.e., the typical type of construction in Ghana), most of the earthquake load is generated at the floor level, and then transferred to the beams, and then to the columns, for onward transmission to the foundations, and eventually the ground. Capacity design requires all possible failure modes of elements lower down the load path to be prevented from occurring before those higher up the load path have occurred. That is to say that, foundations must be stronger than columns, which must in turn be stronger than beams. The beams in a building must therefore fail before the columns do. This requirement is not easily achieved in design due to various inter-related factors, and could require a number of iterations before a final suitable design is obtained.
In addition to the above, the locations where damage is expected to occur are carefully detailed to ensure that they respond in a ductile manner. Damage, therefore, occurs at pre-defined locations in the building that are designed to be ductile. This consequently leads to the entire building responding in a ductile manner. Following the rules of capacity design therefore ensures that a building has enough strength and ductility to respond in a stable manner.
It must, however, be emphasized that buildings designed to be ductile require significant quality control and quality assurance during construction. This is because the ductility of a structure is very much affected by way it is constructed. For example, if a building is designed assuming steel of high ductility, but during construction steel with a lower ductility is used, a brittle-type building response could result even though the structure has been designed to be ductile.
Conclusions:
Going back to the story of the SE and EIT, in the context of seismic design, the advice given by the SE could be described as a flagrant disregard for the concept of capacity design. Interestingly, the client who felt satisfied with the design could in actual fact loose his/her life in the event of a major earthquake. As a final word, stronger buildings are not always safe; however, strong ductile buildings are the safest buildings that could be built on this planet.
Nii Kwashie Allotey, Ph.D., P.Eng., Email: nii50@inbox.com
