Masonry Retrofit


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Masonry Retrofit

  1. 1. GUIDELINES FOR SEISMIC RETROFIT OF EXISTING BUILDINGS Chapter 1. Seismic strengthening Provisions for Un-reinforced Masonry Bearing Wall Buildings. Presented by Doc X. Nghiem June 15, 2002 Introduction Since the conception of LA Division 88 (ASD ELF Method) in 1981, and LA RGA-87 (ASD version of ABK for Zone 4), there have been many changes to the code for new buildings: n Seismic load has increased for soil effect and proximity of faults. n Strength design has been introduced with the 1977building code. n The need for URM retrofit need has spread to other zones other than zone 4. n Experience was gained from recent earthquakes .
  2. 2. Introduction (Cont.) Ch.1 of 2001 GSREB has been updated to include the above changes. It calls for the selection of 1 of 2 procedures: n The General Procedure (GP), based on a strength design ELF method of IBC 2000, using 75% of new buildings seismic load. n The Special Procedure (SP), a limit state design method based on the original ABK research on dynamic behavior of URM buildings with flexible diaphragms. Sec.106. Materials Requirements n Table 1-D - Strength Values for ExistingMaterials or configuration of Materials. n Table 1-E – Strength Values of New Materials used in Conjunction with Existing Construction. n The materials values were obtained from combined static and dynamic test data, research, and engineering judgment.
  3. 3. Existing Un-reinforced Masonry Lay-up of solid multi-wythe brick masonry: n Bond with header bricks is required between wythes of solid brick walls. n Wythe not bonded with headers is veneer. Veneer attachment shall be investigated. n Veneer is not included in the thickness of the wall for h/t calculations. Lay up of other masonry: n Running bond for hollow concrete or clay blocks. Sec 106.3.3 Testing of Masonry n Mortar tests: In-Place Shear Test. n In-Place shear test of mortar bed joint of the outer wythe. n Record Vtest at first movement of test brick. n Record collar joint % coverage. n Record test location for calculation of overburden.
  4. 4. Alternative tests of Masonry n 1. Tensile splitting strength of 8” diam sample core: n An intersection of mortar head and bed joints shall be at the center of the core. n Tensile splitting strength is determined per ASTM C 476: fsp = 2P/pa n
  5. 5. Alternative Tests of Masonry n 2. Tensile splitting strength for hollow unit masonry with through the wall units: n Sawn 18” square sample tested with diagonal in the vertical position n Tensile splitting strength per ASTM E519: fsp = 0.494P/a n Location of Tests n Representative of mortar condition throughout the building. ( Class, Weather..) n Determined by the engineer. n Recorded by the testing agency.
  6. 6. Minimum quality of Mortar n Mortar shear test value for each test: vto = (V test/A b) – PD + L n Mortar shear strength v t is the value exceeded by 80% of v to . (Mean - s) n URM walls with v t < 30 psi are not acceptable. Repointing and retest, or replacement is required. Minimum quality of Masonry n Minimum average value of tensile splitting strength f sp= 50 psi n Repoint and retest, or replace if < 50 psi
  7. 7. Sec 108. Design Strengths n Refer to Table 1-D and Table 1-E for values for existing material and combinations of existing and new materials. n Strengths not specified in this chapter or the Building Code may be as specified by NEHRP Recommended Provisions. n Capacity Reduction factors are not used. n New materials not specified shall be substantiated by research test data. Masonry shear Strength n Determined from in-place shear test: vm = 0.56 v t + 0.75 P D/A <= 100 psi n Determined from tensile splitting tests: v m = 0.8 fsp + 0.5 P D/A Other Design Strengths n Masonry compression: 300 psi n Masonry tension : 0 n Foundation strength without geotech investigation: n New total DL may be increased over existing DL by 25%. n New DL & LL may be increased over existing DL & LL by 50%.
  8. 8. Sec 109 Analysis And Design Procedure. n 1. Elements required to be analyzed: Refer to Table 1-A. n 2. Selection of Procedure: n General Procedure (GP) is used to analyze buildings with rigid diaphragms and flexible diaphragms. n GP is merely the ELF method of the current code. n One deviation: Rocking of masonry piers is allowed per Sec.112. Analysis and Design (cont.) n Special Procedure (SP) is used to analyze buildings with flexible diaphragms when applicable. SP is based on dynamic testing of diaphragms and URM walls responses. n There is no redistribution of calculated base shear similar to Building Code. n Instead, SP uses concept of building element response.The element response shear is an upper bound or yield capacity, such as diaphragm v u.
  9. 9. Analysis and Design (cont.) n The summation of the response of the elements will constitutes a base shear. n When the diaphragm is yielding, large deflections may result in instability of out-of- plane walls. Cross-walls are used to reduce the diaphragm deflection. n In flexible diaphragms that stay near elastic, yielding cross-walls also dampen the diaphragm response and allow higher h/t ratios for out-of-plane walls. GP vs SP
  10. 10. Sec 110. General Procedure n Sec. 110.1 Seismic loading is determined at 75% of Building Code: V = 0.75 SDS / R S DS from IBC = 2.5 C a of UBC S D1 from IBC = Cv of UBC R = 1.5 from IBC for plain masonry n Base shear is distributed according to the Building Code. n Sec.110.2. Elements of Structures: n Diaphragms and their connection to shear walls shall be analyzed and designed per the current Building Code. n Sec.110.3. Stability of out-of-plane walls: URM walls with h/t ratios < set forth in Table 1-B need not be analyzed for out-of-plane loading. n Sec.110.5. Redundancy factor = 1, and vertical component of earthquake force E v=0. n Sec.110.4. In-plane analysis of URM walls is according to Sec 112 - same as SP- using the code distributed story shear.
  11. 11. Sec 111. Special Procedure n Limits for the application of S.P.: n Flexible diaphragms at all levels above base of structure. n Vertical lateral force resisting system consisting predominantly of masonry or concrete shear walls. n Minimum 2 lines of vertical lateral force resistance parallel to each axis of the building. n Lateral Forces on Elements of Structures: n Same as GP, 110.2 to 110.5, except: n Allowable h/t is determined by dynamic behavior of diaphragm (zone 1,2 or 3 of Fig.1-1). n Diaphragm deflection control provisions of 111.4 apply.
  12. 12. Cross-Walls n Sec 111.3. Cross-walls Definition: n Wood framed with materials of Table 1-D&E. n Designed to yield. n Couple diaphragms together and to ground. n Dampen the diaphragm response, hence reduce diaphragm amplification effects. n Spaced no more than 40.' Minimum capacity required. n Full story height and placed in each story of the building. Exceptions: n 1. Cross-walls need not be provided at all levels if used only to couple the roof to the diaphragm below. n 2. Cross-walls need not be continuous to grade under first floor crawl space if: n Diaphragm is anchored to foundations for shear and tension. n There are minimum cross-walls coupling the diaphragm to the foundations. n DCR of the diaphragm sections within cross-walls do not exceed 2.5.
  13. 13. Cross-Walls (cont.) n 111.3.2 Cross-wall Capacity: n Sum of cross-walls shear capacities within any 40 ft of the diaphragm span > = 30% of the stronger diaphragm shear capacity. n Existing Cross-walls n h/D of 1.5 to 1. n Existing connections need not be checked if cross-wall extends from floor to floor. n New Cross-walls n Connections to diaphragms to develop cross- wall capacity. n Design for OTM. OTM need not be cumulative for more than 2 stories. n Steel moment frame may be cross-wall.
  14. 14. 111.4. Wood Diaphragms. Displacement Control. n Calculate DCR for each diaphragm level, starting from the roof level n Diaphragms are acceptable if the point DCR-Span is located within the curve of Fig.1-1. Diaphragm Displacement Control n Demand Capacity Ratios calculations : n 1. Any diaphragm w/o X-walls immediately above and below: DCR= 2.1 S D1 W d / 2vuD n 2. Roof of one story with X-walls or roof coupled with X-walls to diaphragm below: DCR= 2.1 S D1 W d / (Sv uD + V cb ) n 3. Multistory building with x-walls at all levels: DCR= 2.1 SD1SW d / (SSvuD + V cb) n 4. Roof and diaphragm immediately below if coupled by x-walls: DCR= 2.1 SD1SW d / SvuD
  15. 15. Diaphragms (cont.) n Diaphragm chords need not be checked. n Collectors need be designed. n Diaphragm openings reduces the depth D if within the end quarter of diaphragm span. 111.5. Diaphragm Edge Shear Transfer to Shear Walls n Shear connectors at each edge of a diaphragm is designed for the lesser of: V = 1.2 S D1 C p W d C p from Table 1-C is dependent on diaphragm make up. or V = vu D
  16. 16. 111.6. Shear Wall in-Plane Loading n 1. Wall Story Force at any diaphragm level: F wx = 0.8 S D1 (W wx + W d/2) Not to exceed: F wx = 0.8 S D1 W wx + v uD n 2. Wall Story Shear: V wx = S F wx n Shear wall analysis per Sec.112, same as for GP. n Moment frames used in place of shear walls shall have story drifts limited to 0.015 with further restrictions per Sec.112.4.2.
  17. 17. 111.7. Out-of-Plane Forces: Stability of URM Walls n h/t ratios for use in Table 1-B are determined from Fig 1-1: n Region 1: Diaphragm is an elastic amplifier, hence cross-walls are effective. Use h/t ratio for “buildings with cross-walls” if qualifying cross-walls are present in all stories. n Region 2: Diaphragm is heavily loaded and yielding, and already damping the EQ force: Use h/t ratio for “buildings with cross-walls” whether or not qualifying cross-walls are present. n Region 3: Diaphragm is strong and lightly loaded. Cross-walls are not displaced enough to yield: Use h/t ratio for “all other buildings” whether or not qualifying cross-walls are present. n Walls with diaphragms in different regions: n When DCR of the diaphragm above and below the level of consideration result in different regions of Fig 1-1, use the lesser h/t.
  18. 18. Open Front Procedure n Single story. n Open on one side. n Cross-walls parallel to open front. n Use the method of mirror. n Effective diaphragm span and DCR for use in Fig 1-1: Li = 2[(W w/W d)L + L] DCR = 2.12SD1(W d + W w)/[(vuD) + V cb] n Note: If cross wall is not at the open front, another DCR check is made for the cantilevered portion.
  19. 19. Sec.112. Analysis and Design - URM Pier Analysis n Applicable to both GP and SP. n In-plane shear force at any level of a shear wall is distributed to each pier in proportion to the pier stiffness, unless the pier rocks. n Pier rigidity considers shear and neglects flexural rigidity. n Rocking of URM pier is allowed after tension cracks form at the ends of the pier. The axial load on the piers provides restoring resistance to overturning. Pier Analysis n The capacity of a pier is the lesser of the following: n Shear capacity = Allowable shear x pier section area: V a = v mA/1.5 n Restoring rocking capacity: For piers: V r = 0.9P DD/H For solid walls:V r = 0.9(P D + 0.5P w)D/H Shear vs Rocking
  20. 20. Pier Analysis n Rocking controlled mode: n When V r < V a for each pier, the story shear V wx is distributed to each pier in proportion to PDD/H. n The wall will be rocking safely on its piers if: 0.7 V wx < V r Shear Controlled Mode: n When Va < Vr in at least one pier, the story shear is distributed in proportion to D/H. n Use flow chart of Figure 1-2 as a design aid. n The calculated shear in the piers must not exceed pier capacities as defined above or the wall must be strengthened.
  21. 21. Analysis and design (cont.) n Plywood sheathed shear walls allowed for buildings with flexible diaphragm analyzed with SP, Sec.111. n Plywood shear walls may not share lateral forces with other materials along same line of resistance. n Moment resisting frames may not be used in line with URM wall unless the wall has piers with adequate rocking capacity. The story drift ratio shall be 0.0075. Sec.113. Detailed System Design Requirements n Wall anchorage at floor and roof levels. n Minimum wall anchorage at 0.9SD1 x tributary weight or 200 plf. n Anchors at 6Ft max spacing, 2Ft from corner. n Diaphragm shear transfer with shear bolts at 6Ft max. n Collectors are required. n Ties and continuity per Building Code. Detailed System (cont.) n Wall Bracing n Parapets n Veneer n Truss and beam independent secondary support