From the beginning of the designprocess, the earthquake resistance of a building have to considered. It is much more difficult and rarely satisfactory to achieve this afterwards. If the architect requires an asymmetric building, it can still be analysed and designed to be earthquake resistant (however this is outside the scope of this post). But, in many cases a wise re-arranging of elements of structures results a safe structures.
Avoid Structural Weaknesses in the Building Configuration
Try to achieve:
1. Continuous load paths from roof to foundations
2. Moderate dimensions of structural members i.e. avoid very slender columns, beams and walls (minimum dimensions are given in codes, texts and guidelines).
3. Good connections between all elements, especially heavy elements
4. Robust connections between structure and foundations
5. Lightweight roof and other elements in upper portion of building
6. Transfer of horizontal loads e.g. by use of floor diaphragms
7. Good separation between adjacent buildings to prevent pounding
8. Eliminate structural weaknesses such as stairs acting as struts (which redistribute seismic loads)
9. Robust connections from main structure to dormer roofs, gables, other decorative features, parapets and chimneys.
a. Soft storeys, i.e. a storey, often the ground floor, which is considerably less stiff than the other floors
b. Short column effect; where stiff part height infill panels concentrate the loads in short columns
c. Stiff non-structural elements that attract loads - this results in different loads paths to those assumed in design
d. Asymmetry in plan leading to torsional eccentricity
e. Asymmetry in elevation
f. Pounding from neighbouring building, especially when floor heights do not coincide
g. Large openings (doors and windows) adjacent to the corners of the building
h. Large openings in diaphragm walls
i. Poor connections between floor slabs and beams
j. Concrete staircases which are connected to slabs at top and bottom; they must be connected at one level and allowed to slide at the other
k. Heavy, weak and brittle materials in general
l. Heavy roofs, especially in non-engineered buildings
m. Drainpipes placed within slender columns –this practice is only acceptable if columns are sufficiently stout.
Building must have sufficient separation to ensure they do not pound together and damage each
other in an earthquake.
Generally if framed buildings are separated by at least 0.01 H, pounding will not be a problem. (e.g. two 6-metre tall buildings should be separated by 60mm.) If the building floors are not aligned to within 20% vertical separation should be increased to 0.0125 H. For stiff shear wall buildings the separation distances can be reduced to 0.005 H and 0.0061 H respectively.
Other factors can affect the overall performance in an earthquake.
A. All exterior walls must be design to carry face loads (wind loads).
B. Lintels over doors and windows must be properly designed.
Concept Design Report
We recommend preparing a concept design report. This should describe:
1. The primary structural systems
2. The lateral load resisting system (shear walls, moment-resisting frames or braced frames)
3. The loads used (seismic zone, wind zone, live loads)
4. The soil conditions etc.
5. Any potential structural weaknesses and note possible improvements.