How Architectural Features Affect Seismic Resistance of Buildings?
Madeh Izat Hamakareem
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There are several architectural features that critically influence the seismic resistance capacity of structures. Building architectural features include overall shape, size and geometry, in addition to how the earthquake forces are carried to the ground. Each of these architectural features has significant bearing on the performance of the building during earthquakes.Regarding the importance of building configuration, Henry Degenkolb said that “If we have a poor configuration to start with, all the engineer can do is to provide a band-aid - improve a basically poor solution as best as he can. Conversely, if we start-off with a good configuration and reasonable framing system, even a poor engineer cannot harm its ultimate performance too much.”Therefore, at the planning stage itself, architects and structural engineers must work together in order to avoid undesired architectural features and come up with good building configuration. When irregular features are included in buildings, a considerably higher level of engineering effort is required in the structural design and yet the building may not be as good as one with simple architectural features.
How Architectural Features Affect Seismic Resistance of Structures?
Size of Buildings
Horizontal Layout of Buildings
Vertical Layout of Buildings
Adjacency of Buildings
1. Size of Buildings
In tall buildings with large height-to-base size ratio (as shown in Fig.1.a), the horizontal movement of the floors during ground shaking is large which detrimentally affect the stability of a structure, and it increases overturn tendency. With regard to short but very long buildings (as in Fig.1.b), the damaging effects during earthquake shaking are many.
Additionally, in buildings with large plan area like warehouses (Fig.1.c), the horizontal seismic forces can be excessive to be carried by columns and walls. So, the aforementioned sizes of buildings are highly likely to show poor performance during earthquakes.
Fig. 1: Buildings with one of Their Overall Sizes Much Larger or Much Smaller Than the Other two, Do not Perform Well During Earthquakes
Fig. 2: Toppling of Tall Building
2. Horizontal Layout of Buildings
In general, buildings with simple geometry in plan (Fig.3.a) have performed well during strong earthquakes. Buildings with re-entrant corners, like those U, V, H and + shaped in plan (as illustrated in Fig.3.b), have sustained significant damage.
Frequently, the bad effects of interior corners in the plan of buildings can be avoided by making the buildings in two parts. For instance, an L-shaped plan can be broken up into two rectangular plan shapes using a separation joint at the junction (Fig.3.c).
Sometimes, simple plan cannot be used as the only strategy for improving performance of buildings during earthquake, but other factors would play their role as well. For example, simple plan structures with unequal distribution of columns/walls in plan are expected to suffer significant damages during earthquakes.
Buildings with such features tend to twist during earthquake shaking. That is why both architectural and structural engineer need to cooperate to design high seismic resistance structure.
Fig. 3: Simple Plan Shape Buildings Do Well During Earthquakes
3. Vertical Layout of Buildings
The earthquake forces developed at different floor levels in a building need to be brought down along the height to the ground by the shortest path. Any deviation or discontinuity in the load transfer path lead to poor seismic performance of the building. There are several vertical architectural and structural features that decline seismic capacity of buildings:
Buildings with vertical setbacks such as hotel buildings with a few storys wider than the rest cause a sudden jump in earthquake forces at the level of discontinuity, as illustrated in Fig.4.
Fig. 4: Buildings with Vertical Setback
Weak or Flexible Story
Buildings that have fewer columns or walls in a particular story or with unusually tall story as shown in Fig.5, tend to damage or collapse which is initiated in that story.
Fig. 4: Buildings with Soft Storey
Unequal Column Height Along Slopes
Buildings on sloppy ground have unequal height columns along the slope, which causes ill effects like twisting and damage in shorter columns as can be noticed in Fig.6.
Fig. 6: Buildings on Sloppy Ground have Long and Short Columns Which May Twist During Earthquake
Hanging or Floating Columns
Buildings with columns that hang or float on beams at an intermediate story and do not go all the way to the foundation, have discontinuities in the load transfer path as can be seen in Fig. 7. Some buildings have reinforced concrete walls to carry the earthquake loads to the foundation. Buildings, in which these walls do not go all the way to the ground but stop at an upper level, are liable to get severely damaged during earthquakes.
Fig. 7: Buildings with Hanging or Floating Columns
4. Adjacency of Buildings
When two buildings are too close to each other, they may pound on each other during strong shaking. This collision exacerbates as building height increases. Moreover, when building heights do not match (Fig.8), the roof of the shorter building may pound at the mid-height of the column of the taller one; this can be very dangerous.
Fig. 8: Pounding can occur between adjoining buildings due to horizontal vibrations of the two buildings
Fig. 9: Pounding of Adjacent Buildings During Earthquakes