Seismic Strengthening Details

A thorough understanding of existing construction and seismic retrofit objectives acceptable to owners and to the building official is an important consideration before a seismic retrofit is undertaken. The importance of considering global and elemental deformations at expected levels of seismic forces, not at code or design levels, cannot be overstressed. This is because even with the use of amplification factors, the deformations are at best an approximation, particularly when applied to complex multistory and multidegree-of-free-dom systems. It should be kept in mind that detailing in existing buildings often does not meet the requirements of new construction, and that the strength and stiffness of existing elements may not be comparable with new upgraded systems and elements. Thus, verification of elements for deformation compatibility becomes even more important. This criterion is secondary only to the requirement of providing a continuous load path that is sufficiently stiff and strong to resist realistic earthquake forces. Suggested rehabilitation measures listed by deficiencies are given in subsequent paragraphs.

Add elements to complete the load path. This may require adding new shear walls or frames to fill gaps in existing shear walls or frames that are not continued to the foundation. It also may require the addition of elements throughout the building to pick up loads from diaphragms that have no path into existing vertical elements.

2. Redundancy.

Add new lateral-force-resisting elements in locations where the failure of a single element will cause an instability in the building. The added lateral-force-resisting elements should be of comparable stiffness as the elements they are supplementing.

3. Vertical irregularities.

Provide new vertical lateral-force-resisting elements to eliminate vertical irregularity. For weak stories, soft stories, and vertical discontinuities, add new elements of the existing type.

4. Plan irregularities.

Add lateral-force-resisting bracing elements that will support major diaphragm segments in a balanced manner. Verify whether it is possible to allow the irregularity to remain and instead strengthen those structural elements that are overstressed.

5. Adjacent buildings.

Add braced frames or shear walls to one or both buildings to reduce the expected drifts to acceptable levels. With separate structures in a single building complex, it may be possible to tie them together structurally to force them to respond as a single unit. The relative stiffness of each and the resulting force interactions must be determined to ensure that additional deficiencies are not created. Pounding can also be eliminated by demolishing a portion of one building to increase the separation.

6. Lateral load path at pile caps.

Typically, deficiencies in the load path at the pile caps are not a life safety concern. However, if it is determined that there is a strong possibility of a life safety hazard, piles and pile caps may be modified, supplemented, repaired, or, in the most severe condition, replaced in their entirety.

7. Deflection compatibility.

Add vertical lateral-force-resisting elements to decrease the drift demand on the columns, or increase ductility of the columns. Jacketing the columns with steel or concrete is one way to increase their ductility.

The most direct mitigation approach is to add properly placed and distributed stiffening elements—new moment frames, braced frames, or shear walls—that can reduce the interstory drifts to acceptable levels. Alternatively, the addition of energy dissipation devices to the system may reduce the drift.

9. Noncompact members.

Noncompact members can be made compact by adding steel plates. Lateral bracing can be added to reduce member unbraced lengths. Stiffening elements (e.g., braced frames, shear walls, or additional moment frames) can be added throughout the building to reduce the expected frame demands.

10. Strong column-weak beam.

Steel plates can be added, or a steel column can be made composite by enclosing it with reinforced concrete, to increase the strength of the steel columns beyond that of the beams to eliminate this issue. Stiffening elements can be added to reduce the expected frame demands.

11. Connections.

Add a stiffer lateral-force-resisting system to reduce the expected rotation demands. Connections can be modified by adding flange cover plates, vertical ribs, haunches, or brackets, or by removing beam flange material to initiate yielding away from the connection location (e.g., via a pattern of drilled holes or the cutting out of flange material). Partial penetration splices, which may become more vulnerable for conditions where the beam-column connections are modified to be more ductile, can be modified by adding plates and/or welds. Moment-resisting connection capacity can be increased by adding cover plates or haunches.

12. Frame and nonductile concerns.

Add properly placed and distributed stiffening elements, such as shear walls, to supplement the moment frame system with a new lateral-force-resisting system. For eccentric joints, columns and beams may be jacketed to reduce the effective eccentricity. Jackets may be also be provided for shear critical columns.

• Short captive columns.

Columns may be jacketed with steel or concrete such that they can resist the expected forces and drifts. Alternatively, the expected story drifts can be reduced throughout the building by infilling openings or adding shear walls.

13. Cast-in-place concrete shear walls.

Add new shear walls and/or strengthen the existing walls to satisfy seismic demand criteria. New and strengthened walls must form a complete, balanced, and properly detailed lateral-force-resisting system for the building. Special care is needed to ensure that the connection of the new walls to the existing diaphragm is appropriate and of sufficient strength such that yielding will occur in the wall first. All shear walls must have sufficient shear and overturning resistance.

Lengthening or adding shear walls can reduce overturning demand.

Strengthen the walls to eliminate the need to rely on the coupling beam. The beam should be jacketed only as a means of controlling debris. If possible, the existing opening should be infilled.

• Boundary component detailing.

Splices may be improved by welding bars together after exposing them. The shear transfer mechanism can be improved by adding steel studs and jacketing the boundary components.

14. Steel Braced Frames

• System deficiency. If the strength of the braced frames is inadequate, braced bays or shear wall panels can be added. The resulting lateral-force-resisting system must form a well-balanced system of braced frames that do not fail at their joints and are properly connected to the floor diaphragms, and whose failure mode is yielding of the braces rather than overturning.

• Stiffness of diagonals. Diagonals with inadequate stiffness should be strengthened using supplemental steel plates, or replaced with a larger or different type of section. Global stiffness can be increased by the addition of braced bays or shear wall panels.

• Chevron or K-bracing. Columns or horizontal girts can be added to support the tension brace when the compression brace buckles, or the bracing can be revised to another system throughout the building. The beam elements can be strengthened with cover plates to provide them with the capacity to fully develop the unbalanced forces created by tension brace yielding.

• Braced-frame connections. Column splices or other braced-frame connections can be strengthened by adding plates and welds to ensure that they are strong enough to develop the connected members. Connection eccentricities that reduce member capacities can be eliminated, or the members can be strengthened to the required level by the addition of properly placed plates. Demand on the existing elements can be reduced by adding braced bays or shear wall panels.

6.9.1. Common Strategies for Seismic Strengthening

Techniques for strengthening or upgrading existing buildings will vary according to the nature and extent of the deficiencies, the configuration of the structural systems, and the structural materials used in construction. The following Figs. 6.11 through 6.25 show the seismic upgrading of typical structural members or systems. They provide guidelines for the application of designers' judgment and ingenuity in addressing specific situations. Many of the details shown are adapted from technical manual TM 5-809-10-12 published by the Departments of the U.S. Army, Navy, and Air Force.

Haunched Wall

Figure 6.11. (N) openings in an (E) 3-story concrete shear wall building. The seismic upgrade consisted of providing concrete overlay to restore shear capacity of walls and adding boundary elements around (N) openings: (a) wall elevation; (b) concrete overlay with (N) beam below (E) slab; (c) concrete overlay with (N) beam above (E) slab; (d) plan detail at (N) boundary element. Note: (E) = existing (N) = new

Figure 6.11. (N) openings in an (E) 3-story concrete shear wall building. The seismic upgrade consisted of providing concrete overlay to restore shear capacity of walls and adding boundary elements around (N) openings: (a) wall elevation; (b) concrete overlay with (N) beam below (E) slab; (c) concrete overlay with (N) beam above (E) slab; (d) plan detail at (N) boundary element. Note: (E) = existing (N) = new

Provide temp. holes max. size 12" x 12" as req'd for placing conc. typ. Do not cut (E) reinf. min. spacing of temp. holes 10"-0"±. Patch holes with 4,000 psi conc.

Conc. beam per elev.

4" Conc. overlay for full length of (E) wall

Roughen face of (E) wall & slab 1/4" amplitude min.

Conc. beam per elev.

4" Conc. overlay for full length of (E) wall

Roughen face of (E) wall & slab 1/4" amplitude min.

Figure 6.11. (Continued)

Remove (E) wall

Remove (E) wall

Detail 3 (d)

Figure 6.11. (Continued)

Figure 6.12. Seismic upgrade of a concrete hospital building with an external concrete moment frame. Modifications are restricted to the periphery of the building to keep the building operational with minimal interference to its functionality. (a) plan showing (N) foundations, (N) concrete overlay in the transverse direction, and (N) moment frames in the longitudinal direction.

Figure 6.12. Seismic upgrade of a concrete hospital building with an external concrete moment frame. Modifications are restricted to the periphery of the building to keep the building operational with minimal interference to its functionality. (a) plan showing (N) foundations, (N) concrete overlay in the transverse direction, and (N) moment frames in the longitudinal direction.

Concrete Shear Wall Overlay
Enlorqed plan at (N) coupling beam ond shear wall overlay.

Figure 6.12b. Enlarged plan at (N) coupling beam and shear wall overlay.

Figure 6.12c. Section through longitudinal frame.
Transverse Reinforcement Walls
Figure 6.12d. Section through transverse wall.
Detail A

Figure 6.12e. Connection between (N) and (E) frame.

Strengthening Concrete Wall With Opening
Figure 6.13. Strengthening of existing connecting beams in reinforced concrete walls.
Column Pile Cap Foundation Connection
Figure 6.14. Upgrading of an existing pile foundation. Add additional piles or piers, remove, replace, or enlarge existing pile caps.
Concrete Frame Building Section
Figure 6.15. Strengthening of an existing concrete frame building with a reinforced concrete shear wall.
Reinf Section Concrete Shear Wall
Figure 6.15. (Continued) Section 9-9 (when new wall extends above existing slab).

New reinf. concrete, cast-in-place or pneumatically placed

Wall anchors, epoxy grouted in drilled holes. Not to exceed 3'-0 ctrs. ea. way

Wall anchors, epoxy grouted in drilled holes. Not to exceed 3'-0 ctrs. ea. way

Exist. reinf. concrete wall or pier

Clean and roughen exist. wall surface

Figure 6.16. Strengthening of existing reinforced concrete walls or piers.

Exist. reinf. concrete wall or pier

Clean and roughen exist. wall surface

Figure 6.16. Strengthening of existing reinforced concrete walls or piers.

Shear Wall For Existing Brick Walls
Figure 6.17. Strengthening of existing reinforced concrete walls by filling in of openings.
Column Ties
Figure 6.18. Jacketing of circular column.
Cable Bracing System
Figure 6.19. Braced structural steel buttresses to strengthen an existing reinforced concrete building.
Doubler Plate Strengthening Plate

Modified connection

Figure 6.20. Modification of an existing simple beam connection to a moment connection.

Modified connection

Figure 6.20. Modification of an existing simple beam connection to a moment connection.

Figure 6.21. Strengthening of existing bracing.

Steel channels back to back. Size as required.

Section a-a

Existing structural

Section a-a

Existing structural

Figure 6.22. Strengthening of an existing building with eccentric bracing.

Steel channels back to back. Size as required.

Figure 6.22. Strengthening of an existing building with eccentric bracing.

Figure 6.23. Strengthening of existing columns.

Building plan showing locations of (N) steel props

Externally Braced Wind Turbine
Figure 6.24. (a) Building plan showing location of (N) steel props; (b) section A; elevation of (N) steel prop.
Figure 6.25. Upgrading an existing building with external frames.
Figure 6.25. (Continued)
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