GUIDELINES FOR SEISMIC RETROFITOF EXISTING BUILDINGS

Chapter 1. Seismic strengthening Provisions for Un-reinforced Masonry Bearing Wall Buildings.

Presented by Doc X. NghiemJune 15, 2002

Introduction

Since the conception of LA Division 88 (ASD ELFMethod) in 1981, and LA RGA-87 (ASD version ofABK for Zone 4), there have been many changesto the code for new buildings:

n Seismic load has increased for soil effect andproximity of faults.

n Strength design has been introduced with the1977building code.

n The need for URM retrofit need has spread to otherzones other than zone 4.

n Experience was gained from recent earthquakes .

Sec.106. Materials Requirements

n Table 1-D - Strength Values forExistingMaterials or configuration of Materials.

n Table 1-E – Strength Values of New Materialsused in Conjunction with Existing Construction.

n The materials values were obtained fromcombined static and dynamic test data,research, and engineering judgment.

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 strengthdesign ELF method of IBC 2000, using 75% ofnew buildings seismic load.

n The Special Procedure (SP), a limit state designmethod based on the original ABK research ondynamic behavior of URM buildings with flexiblediaphragms.

Existing Un-reinforced Masonry

Lay-up of solid multi-wythe brick masonry:

n Bond with header bricks is required betweenwythes 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 ofthe wall for h/t calculations.

Lay up of other masonry:

n Running bond for hollow concrete or clayblocks.

Sec 106.3.3 Testing of Masonry

n Mortar tests: In-Place Shear Test.

n In-Place shear test of mortar bed joint of theouter wythe.

n Record Vtest at first movement of test brick.

n Record collar joint % coverage.

n Record test location for calculation ofoverburden.

Alternative tests of Masonry

n 1. Tensile splitting strength of 8” diamsample core:

n An intersection of mortar head and bed jointsshall be at the center of the core.

n Tensile splitting strength is determined perASTM C 476:

fsp = 2P/pan

Alternative Tests of Masonry

n 2. Tensile splitting strength for hollow unitmasonry with through the wall units:

n Sawn 18” square sample tested with diagonal inthe vertical position

n Tensile splitting strength per ASTM E519:

fsp = 0.494P/an

Location of Tests

n Representative of mortar conditionthroughout the building. ( Class, Weather..)

n Determined by the engineer.

n Recorded by the testing agency.

Minimum quality of Mortar

n Mortar shear test value for each test:vto = (Vtest/Ab) – PD + L

n Mortar shear strength vt is the value exceededby 80% of vto . (Mean - s)

n URM walls with vt < 30 psi are not acceptable. Repointing and retest, or replacement isrequired.

Minimum quality of Masonry

n Minimum average value of tensile splittingstrength fsp= 50 psi

n Repoint and retest, or replace if < 50 psi

Sec 108. Design Strengths

n Refer to Table 1-D and Table 1-E for values forexisting material and combinations of existingand new materials.

n Strengths not specified in this chapter or theBuilding Code may be as specified by NEHRPRecommended Provisions.

n Capacity Reduction factors are not used.

n New materials not specified shall besubstantiated by research test data.

Masonry shear Strength

n Determined from in-place shear test:vm = 0.56 vt + 0.75 PD/A <= 100 psi

n Determined from tensile splitting tests: vm = 0.8 fsp + 0.5 PD/A

Other Design Strengths

n Masonry compression: 300 psi

n Masonry tension : 0

n Foundation strength without geotechinvestigation: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%.

Sec 109 Analysis And DesignProcedure.

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 buildingswith rigid diaphragms and flexible diaphragms.

n GP is merely the ELF method of the current code.

nOne deviation: Rocking of masonry piers is allowedper Sec.112.

Analysis and Design (cont.)

nSpecial Procedure (SP) is used to analyzebuildings with flexible diaphragms whenapplicable. SP is based on dynamic testing ofdiaphragms and URM walls responses.

nThere is no redistribution of calculated baseshear similar to Building Code.

n Instead, SP uses concept of building elementresponse.The element response shear is anupper bound or yield capacity, such asdiaphragm vu.

Analysis and Design (cont.)

n The summation of the response of theelements will constitutes a base shear.

nWhen the diaphragm is yielding, largedeflections may result in instability of out-of-plane walls. Cross-walls are used to reducethe diaphragm deflection.

n In flexible diaphragms that stay near elastic,yielding cross-walls also dampen thediaphragm response and allow higher h/tratios for out-of-plane walls.

GP vs SP

Sec 110. General Procedure

n Sec. 110.1 Seismic loading is determinedat 75% of Building Code:

V = 0.75 SDS / R

SDS from IBC = 2.5 Ca of UBCSD1 from IBC = Cv of UBCR = 1.5 from IBC for plain masonry

n Base shear is distributed according to theBuilding Code.

n Sec.110.2. Elements of Structures:

n Diaphragms and their connection to shear wallsshall be analyzed and designed per the currentBuilding Code.

n Sec.110.3. Stability of out-of-plane walls: URM walls with h/t ratios < set forth in Table1-B need not be analyzed for out-of-planeloading.

n Sec.110.5. Redundancy factor = 1, and verticalcomponent of earthquake force Ev=0.

n Sec.110.4. In-plane analysis of URM walls isaccording to Sec 112 - same as SP- using the code distributed story shear.

Sec 111. Special Procedure

n Limits for the application of S.P.:

n Flexible diaphragms at all levels above baseof structure.

n Vertical lateral force resisting systemconsisting predominantly of masonry orconcrete shear walls.

n Minimum 2 lines of vertical lateral forceresistance parallel to each axis of thebuilding.

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 behaviorof diaphragm (zone 1,2 or 3 of Fig.1-1).

n Diaphragm deflection control provisions of 111.4apply.

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 reducediaphragm amplification effects.

n Spaced no more than 40.' Minimum capacityrequired.

n Full story height and placed in each story of thebuilding.

Exceptions:

n 1. Cross-walls need not be provided at all levels if usedonly to couple the roof to the diaphragm below.

n 2. Cross-walls need not be continuous to grade underfirst floor crawl space if:

n Diaphragm is anchored to foundations for shear andtension.

n There are minimum cross-walls coupling thediaphragm to the foundations.

n DCR of the diaphragm sections within cross-walls donot exceed 2.5.

Cross-Walls (cont.)

n 111.3.2 Cross-wall Capacity:

n Sum of cross-walls shear capacities within any 40ft of the diaphragm span > = 30% of thestronger 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 cumulativefor more than 2 stories.

n Steel moment frame may be cross-wall.

Displacement Control.

n Calculate DCR for eachdiaphragm level, starting from the roof level

nDiaphragms are acceptableif the point DCR-Span islocated within the curve of Fig.1-1.

111.4. Wood Diaphragms.

Diaphragm Displacement Control

n Demand Capacity Ratios calculations :

n 1. Any diaphragm w/o X-walls immediatelyabove and below:

DCR= 2.1 SD1 Wd / 2vuD

n 2. Roof of one story with X-walls or roof coupledwith X-walls to diaphragm below:

DCR= 2.1 SD1 Wd / (SvuD + Vcb)

n 3. Multistory building with x-walls at all levels:

DCR= 2.1 SD1SWd / (SSvuD + Vcb)

n 4. Roof and diaphragm immediately below if coupled by x-walls:

DCR= 2.1 SD1SWd / SvuD

Diaphragms (cont.)

n Diaphragm chords need not be checked.

n Collectors need be designed.

n Diaphragm openings reduces the depth D if withinthe end quarter of diaphragm span.

111.5. Diaphragm Edge ShearTransfer to Shear Walls

n Shear connectors at each edge of a diaphragm isdesigned for the lesser of:

V = 1.2 SD1 Cp Wd

Cp from Table 1-C is dependent on diaphragm make up.

or V = vu D

111.6. Shear Wall in-Plane Loading

n 1. Wall Story Force at any diaphragm level:Fwx = 0.8 SD1 (Wwx + Wd/2)

Not to exceed:Fwx = 0.8 SD1 Wwx + vuD

n 2. Wall Story Shear:Vwx = S Fwx

n Shear wall analysis per Sec.112, same as for GP.

n Moment frames used in place of shear walls shallhave story drifts limited to 0.015 with furtherrestrictions per Sec.112.4.2.

111.7. Out-of-Plane Forces: Stability of URM Walls

n h/t ratios for use in Table 1-B are determinedfrom 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” ifqualifying 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” whetheror 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 thelevel of consideration result in different regions ofFig 1-1, use the lesser h/t.

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[(Ww/Wd)L + L]DCR = 2.12SD1(Wd + Ww)/[(vuD) + Vcb]

n Note: If cross wall is not at the open front,another DCR check is made for the cantileveredportion.

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 isdistributed to each pier in proportion to the pierstiffness, unless the pier rocks.

n Pier rigidity considers shear and neglects flexuralrigidity.

n Rocking of URM pier is allowed after tensioncracks form at the ends of the pier. The axialload on the piers provides restoring resistance tooverturning.

Pier Analysis

n The capacity of a pier is the lesser of thefollowing:

n Shear capacity = Allowable shear x piersection area:

Va = vmA/1.5

n Restoring rocking capacity:

For piers: Vr = 0.9PDD/H

For solid walls:Vr = 0.9(PD + 0.5Pw)D/H

Shear vs Rocking

Pier Analysis

nRocking controlled mode:

n When Vr < Va for each pier, the storyshear Vwx is distributed to each pier inproportion to PDD/H.

n The wall will be rocking safely on its piers if:

0.7 Vwx < Vr

Shear Controlled Mode:

n When Va < Vr in at least one pier, the storyshear 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 notexceed pier capacities as defined above or thewall must be strengthened.

Analysis and design (cont.)

n Plywood sheathed shear walls allowed forbuildings with flexible diaphragm analyzedwith SP, Sec.111.

n Plywood shear walls may not share lateralforces with other materials along same line of resistance.

n Moment resisting frames may not be used inline with URM wall unless the wall has pierswith 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 tributaryweight or 200 plf.

n Anchors at 6Ft max spacing, 2Ft from corner.

n Diaphragm shear transfer with shear bolts at6Ft max.

n Collectors are required.

n Ties and continuity per Building Code.

Detailed System (cont.)

n Wall Bracingn Parapetsn Veneern Truss and beam independent

secondary support