Apr 2008

Choosing Structural Systems for Earthquakes

Several factors come into play when it comes to designing earthquake protection for a building. These include site selection, building type and the particular building code used. This month, we will discuss considerations for structural design.

Briefly, earthquake forces are generally horizontal (lateral) forces acting on a structure. The design of a building is meant to resist and transfer the lateral forces into vertical forces. This is known as a Lateral Force-Resistant System (LFRS). Vertical forces created by an earthquake are adequately resisted by the structure except in certain cases, which are addressed by the local codes.

The generic performance of a building in an earthquake is dependent upon:

Period of vibration – This is dependent upon the building height, the stiffness of the building and how the building/content weight is distributed in the building.

Building configuration – When considering configuration the main factors are:

Shape: Regular shaped buildings such as squares, rectangles and circles perform better than irregular shaped buildings such as triangles, concave walls (also known as reentrant corners) or generally asymmetrical. During an earthquake, differential motions can vary between the irregular building sections and create local stress concentrations. The irregularity in shape can also create different locations for the building’s center of mass and center of resistance. During an earthquake, the building mass will swing around the center of resistance, creating torsion.

Soft or weak stories: These are stories in a building that are significantly weaker than the adjacent stories. This would normally be found on the first floor of a multi-storied building. Soft stories are more flexible than the other stories, while a weak story is not as strong as the other stories. Without a soft/weak story, the building movement in an earthquake is distributed evenly along all floors. With a soft/weak story the movement is concentrated at that floor with overstressing occurring at the connection to the floor above. This can lead to deformation or total failure of the connections.

Discontinuous shear walls: These are structural walls that do not extend completely along the horizontal or vertical building dimensions. The discontinuity prevents a continuous load path from the roof/floors to the foundation and is a specialized form of a soft story. Earthquake forces can concentrate at the points of discontinuity and cause overstressing, which may lead to deformation or total failure.

Variations in strength and stiffness of building perimeter: The building may be regularly shaped geometrically but not structurally. An example is where stiffer walls are provided on some but not all exterior walls. This causes the horizontal center of mass and center of resistance to be in different places, creating torsion during an earthquake.

Pounding: A building may move (drift) during an earthquake. Adjacent buildings too close together may pound each other and cause architectural and/or structural damage.

The different types of building construction and their performances include:

Lightweight Metal – This is a one-storied prefabricated building using light-gauge steel framing. The exterior walls and roof deck are normally (but not exclusively) metal. Lateral loads are transferred from the building roof deck or roof rod/cable bracing to the building beams and columns. These perform well structurally, but would have greater architectural damage.

Wood Frame – This can be single- or multi-storied structure using wood sheathing and framing. Lateral loads are transferred from the roof and floor sheathing to shear walls sheathed with wood, gypsum board or stucco. Problems arise when the building frame is not adequately bolted to the foundation. Where a wood frame crawl space is created under the first floor, if the short (cripple) walls are not braced properly, they can tip. Shear walls using stucco or gypsum board have not performed as well; subsequent codes have decreased or eliminated their use as shear walls.

Steel Braced Frame – This is commonly a concrete on steel deck floor/roof system and is supported by steel beams and columns (frame). Lateral loads are resisted by the concrete deck (diaphragm) and are transferred to the entire frame. Problems occurred where the beam-column connection was not adequately braced during the 1971 San Fernando Earthquake; this has been addressed in building codes such as the 1973 Edition of the UBC. Cables and rods, which are tension-only bracing have performed poorly and are no longer allowed.

Steel Moment Frame – This is similar in appearance and basic construction to the Steel Braced Frame, though lateral loads are transferred to pre-selected points on the frame and that only some portions of the frame are used to resist the lateral loads. The beam-column connections were originally bolted but moved to welds in the 1950s. After the 1994 Northridge Earthquake several buildings developed brittle cracking at their welded connections. Subsequent changes to the code have been made to mitigate the connection concern.

Rigid Shear Wall / Rigid Diaphragm – Typically this building has reinforced concrete/masonry walls and concrete diaphragms. The concrete diaphragm can be reinforced slabs or poured-on steel decking. Lateral loads are resisted by the diaphragms and transferred to the shear walls. Poor performance has come from buildings with discontinuous shear walls and variations in strength and stiffness of building perimeter.

Rigid Shear Wall / Flexible Diaphragm – This building typically has reinforced concrete/masonry walls and lightweight diaphragms such as wood sheathing or metal decking on wood or steel framing. Lateral loads are resisted by the diaphragm and transferred to the shear walls. Problems occurred where the heavy shear walls were not adequately anchored to the diaphragms and separation occurred. This was most evident during the 1971 San Fernando and 1994 Northridge Earthquakes. Subsequent changes were made to the building codes.

Concrete Moment Frame – This building is reinforced concrete frame with reinforced concrete diaphragms. The lateral load is resisted by the diaphragm and transferred to the frame. Problems have occurred where the reinforcing was not sufficiently detailed, such as insufficient ties to prevent buckling of some reinforcing steel or the use of precast concrete moment frames. This was initially addressed in the 1967 Edition of the UBC and in subsequent editions.

Unreinforced Masonry (URM) – These are buildings that have bearing walls with no steel reinforcement. The bearing walls can be exterior or interior. The most common construction is wood sheathing/frame floors and the URM walls. This type of construction has suffered severe damage during earthquake as the masonry, while strong in compression, has little resistance in tension. After the 1933 Long Beach Earthquake, URM buildings were generally not allowed. Retrofits for URM structures in general have been to prevent building collapse; buildings in areas with a severe earthquake exposure will suffer significant damage whether retrofitted or not.

Risk Logic can help in determining the potential earthquake hazard that may exist to a building and its operations.