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REPAIR ANO REHABILlTATION OF EXISTlNG BUllOINGS

WALTER L. DICKEY Civil & Structural Engineer

Los Angeles, Ca l i[ornia 90012, U. S .A.

REPAIR AND REHABILITATION OF EXISTING BUILDINGS

The need for rehabilitation of old or damaged

buildings has increased greatly in the Western U. S.A .

There have been earthquakes causing damage and the

codes have become more stPingent as research has in­

creased the engineePing knowledge of resistance to

be provided. Long occupation of such structures ,

and quake damage to some, has requirod consider>able

consideration of repair or strengthening.

REPARATION ET RESTAURATION DE BATIMENTS EXISTANTS

La nécessité de restaurer des bâtiments anciens ou

endommagés a fortement augmenté dans l ' Ouest des

Etats- Unis . Les trembZements de terre ont causé des

dégâts et les reglements sont devenus plus stricts à

mesure que ~ recherche apportait un supplément de

connaissances en génie civil concernant la résistance

à prévoir . Une longue occupation de teZles structu­

res et, pour certaines , des dommages dus à des se­

cousses telluriques ont nécessité de prendre sérieu­

sement en considération leur réparation ou leur ren­

forcement .

INSTANDSETZUNG ~STEHENDER GEBAEUDE

Der Instandsetzungsbedarf alter oder beschadigter

Bauten hat in den westlichen Bundesstaaten der USA

stark zugenommen . Erdbeben haben Schaden verursacht

und die Normen sind stronger geworden. ~ die For ­

schung entsprechende Kenntnisse über die Festigkeit

zur Verf ügung gestellt hat. Lange Benützung solcher

Bauten und Schaden durch Erdbeben haben es erforder­

lich gemacht. Reparatur- und Verstarkungsm6glich­

keiten gPÜndlich zu überlegen.

HERSTELLING EN HERBEWOONBAAR MAKEN

VAN BESTAANDE GEBOUWEN

De behoefte aan herbewoonbaar maken van oude of

beschadigde gebouwen is in de laatste jaren in

het westen van de ,USA groter geworden . Als ge­

volg van aardbevingen die schade veroorzaakt heb­

ben zijn de nOPmen strenger geworden omdat de ken­

nis van de weerstand die nodig is door wetenschap­

pelijk onderzoek verbeterd werd. Langdurige be­

woning van sommige gebouwen en aardbevingsschade

aan sommige , heeft de belangstelZing voor herstel­

ling en versteviging doen toenemen .

INTROOUCTION

The following discussion is concerned primarily with the damage that may have occu rred to a structure due to earthquake, blast, or strong winds. It will not be pertinent to buildings too badly damaged to be re­habilitated economically .

The subjects discussed are:

"Verification of the existing masonry properties" "Structural systems and elements and the typical modes of fa il ure"

"Methods of repair or strengthenl ng" ~Oesi gn provisions and cade requirements" "Check list, guide outline f or reference and check "

The last item is a memory jogger and considered as an Appendix .

VERIFICATION OF MASONRY PROPERTIES

General

Some testing may have been done in a previous inves ti­gation . Fo r final design, it wil l be necessary for certain detailed investigation to be made . Tests are outlined for correct determination of vertical bearing capacity , shear forces perpendicular to the pl ane . in­plane shear capacities . bending capacity. etc.

Some of the typical investigative test procedures for evaluation of st ructural capacity are listed.

Sonic

This method has not been used extensively satisfac ­torily in masonry although it has some potential. Masonry is an assemblage of diff~rent elements, a heterogeneous ent i ty rather than homogeneous . The difference between modulii and the frequent contacts between component par t s may produce readings falsely i ndicating "cracks".

Thi s method has some merit and it is r ecommended that further r esea rch be continued in order to develop it. The advantages of non-destructive test methods are great .

Percussion

This method of conc rete testing has not been developed extensively for us e on masonry assemb lages, which con­tain portio ns with different modulii and strength r e ­lations .

El ectromagnetic

Has been utilized for loca tion of interior reinforcing. One simple method utilized is the carpenter's "Stud Finder", rather effective for locating joint rein­forcing . This wi re reinforcing is near the surface Df the joint and can sometimes be detected easily.

"Treasure finders " or"mine detectors " have been suc­cessful . Refinement of such generic equipment has been very successful. One de vice tested in construc­tion could provide:

(1) existence of bars (2) l ocation and direction of bars (3) depth of steel below the surface (4) size of bar or bars

Orilling core samp l es has been used frequently as a

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quick easy method of determining the quality of mason­ry. Although rather simple, it has limitations.

(a) ~t shows, visually, the quality of masonry at one small area only .

(b) It may give arroneous results , If cutting bits are not sharp the specimen may be damaged, especially in masonry with the old more brittle fragile mortar. There may be improper handling.

(c) The core axis is perpendicular to the wall. Compression in this direction will very likel y not show true strength of the material in the direction Df the wall load. It may be tested to shear off a face of the masonry from the interior for a determination of so-called "longitudinal shear", i.e . VQ/lb, not a very critical fact or.

(d) The core may be tested in spl itting as concrete cylinders are.

(e) The core test may determine the joint sliding resistance.

Prisms

The cutting of prisms will gi ve a measure of f' or masonry assemblage strength quite appropriate f~ ver­tical compression . However, it is not an f' to which coefficients may be app li ed for true values ~f shear , especially in cases of old lime mortar .

Shear Testing

The present shear val ues are assumptions of fractions of compressive strength of newe r masonry assemblages and there is wide variation Df test results from average.

Weak mortar wi ll permit l ow values for joint sliding failures though the compression strength may be high.

Therefore , to be more certain of the critical she·ar value, it is recommended that prisms be tested on the diagonal, which will give a measure of the resistance capacity (Schneider, EERC, KPFF, BIA, NCMA, Blume). This wi ll also provide a verification of the sliding resistance .

Modulus of Elasticity

The stiffness will influence deflection and vibration as well as distribution of stress and should be deter­mi ned by test rather than by assumpti on .

General

MASONRY STRUCTURAL ELEMENTS ANO TYPICAL FAILURE MOOES

There are many types of damage that may occur to walls due to over stressing under seismic l oads . Some Df the cracks may be difficult to find because they will be slight and would have occurred in the elastic range, and hence will have closed. One aid to finding them is knowing where they are. One may anticipate the l ocation by noting the structural system and its possible failure.

Some of the types are listed in the guide outline but are described in more detail here in order to assist in location of such damage.

Non-Bearing Walls

Simple masonry walls providing separation of area, in­cluding separation between exterior and interior.

The type that is called "in-filled" consists of mason­ry walls built .between the columns, floors and beams . Although not designed to do so , these frequently

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function as shear walls. These are frequently not tied to the structure although occasionally there are dowels, tying them to the floor.

Since they are between the structural elements, they resist deformation of the panel. For excessive loads in the plane of the wall, some common modes of failure are as shown in the drawing. i.e.:

(1 ) & (5) Local crushing of the corners (2) Joint sliding (3) Diagonal cracking through the head and bed

joints (4) Diagonal crack1ng through the units

They might also form wedge shaped portions while pro­viding restraint to the columns. They may have forced a secondary failure in the column frame member, shown by cracks. These might be quite tight, however, since they may be under considerable compression and diffi­cult to defect.

Dther non-bearing walls or partitions which may not have adequate support at the top and which would be subjected to forces perpendicular to the wall plane, might have rocked on the base forming horizontal cracks at the base. This type, also, might function as the shear wall when between columns, just as the infilled walls functioned, with similar cracks.

Bearing Walls

These are the majority of masonry construction; they are the walls which carry the building loads, con­centrically or eccentrically. In addition, of course, these must resist the lateral loads of wind, blast, earthquake, etc ., perpendicular to their plane, while serving as part of the structural system to resist inplane forces as shear walls .

Due to their carrying vertical load as a prestressing action, there is generally not much tendency for these walls to fail when spanning from floor to cei ling while resisting loads perpendicular to their plane.

These walls may also develop resistance to loads per­pendicular to thelr plane by arching action. This failure would be indicated by cracks and crushing at ends and at center between the supports .

The types of failure will be as for infilled walls, that is, failures perpendicular to the wall shown by horizontal cracks at the center as the wall spans from top to bottom. or vertical cracks as the wall spans to supports at the ends, such as columns or pilasters. In addition, there will be the failures due to inplane ~orces as the bearing wall provides shear resistance to the structure.

Shear Walls

In the case Df heavy overloads , there may be diagonal cracks due to diagonal compression, or diagonal ten­sion, failures which may be through the pattern Df the joints or may be diagonally through the masonry units. In addition, there may be the local compression fail­ures at the corners due to the diagonal racking of a frame, or there may be a sliding on the base or top if the connections were not adequate.

Piers

These are portions of wall between openings in which the loads, vertical and lateral, may be concentrated. Piers may function restrained as cantilevers pinned at the top, or cantilevers fixed·top and bottom, or pinned top and bottom. They may fail in bending due to loads perpendicular to their plane as they span from the top to bottom, with horizontal cracks at the center, although this is not frequento They may func­tion as shear elements in a bearing shear wall situ-

ation with the typical diagonal X cracks. They may fail due to the in-plane shear loads either in bending or in shear or in combination. As discussed, the di­agonal cracks may be through the units or following . the pattern of the mortar head and bed joints. There may be a tension yielding on first one side and then the other during a quake and this tension yielding will cause the compressive stresses to be concent rated at the opposite corner. This compressive stress added to the shear stress wil l cause local toe compression failures.

Moment failure may be due to the compression failure and also due to tension failure Df inadequate steel reinforcing, showing a horizontal crack as the steel had elongated. Whether the steel had stretched beyond its elastic limit may have to be determined. I t may, of course, be closed due to the compressive load from above.

Lintels

The lintels are the portions of the wall over openings. The load assumptions on the portion Df the walls assumed as lintels are subject to considerable j udge­ment. In general, the masonry material will tend to arch over an opening if there is adequate abutment reaction. The load on the lintel is generally slight, like the filler space over a door head, under an arch.

Columns

These are elements occasionally used isolated, or they may be portions of walls to carry vertical loads, or built with the wall to carry the vertical loads as pilasters or to provide buckling stiffness. The most frequent problem is with damage to the top due to end rotation and concentration of load. Early design generally had inadequate provision for this aetion. There are very frequently inadequate ties at the top and craeks would oeeur under eoncentrations of load sueh as girders or beams.

METHDDS DF REPAIR

General

Some of the many are deseribed as to generic method as follows.

The problems of rev1s1ng and rehabilitating existing buildings has certain aspeets different from the ini­tial.design of a building in which there is no.t the free choiee of systems. With existing buildings, the size, type, wall loeations, material and orientation are al ready fixed, which reduees the freedom of choice Df design inherent with new buildings.

AIso, there is the influenee Df history. There may be deterioration of elements. There may be results of ehemieal reaetions. There may be stresses indueed due to long-time foundation settlement or beam end rota­tion. There may have been impact damage as well as the seismie damage. Therefore there may be some need for refinement of design beyond that whieh might be required for new struet ures . Dne must investigate more thoroughly the actual performance Df elements.

Gunite

This 'versatile method received a good impetus in the 1933 Long Beach earthquake. Gunite, or Shoterete, is a method Df app lying a cement sand mix with an impact that assures a good bond . It is a mix with a rather good water eement ratio for good strength and mini­mum shrinkage. This method Df applieation provides exeellent freedom Df shape with good bond and strength.

~v

Diagram showing various failure modes Df in-filled panels, 1,2 , 3 , 4 and 5

RE.SfRAlNING Wr:OCt.) .. .- --- { --- -,. ---- ------.. -- .

I-

i-

r--_ ------------ ---I

•

Alternate mode Df fail ure Df in-filled walls. The pa ne l 1s st r ong enough to provide r estraint to ends Df column by a rest r aining wedge .

5 . c . 1 - 3

5 . c . 1-4

Prestressing

Th ere have been examples in which rods were inserted in dri lled cores, threaded into f oundation anchorages and stressed to an appreciable leveI, preventing ten­sion due to the lateral loadings . Brickwork is e­specially suited for prest ressi ng because it does no t creep nor s hrink the way concrete does to relieve the prestress.

Plaster

Plaster has been used to bond reinforcing mesh to masonry surfaces in rather in teresting methods. It 1s lJery eff ecti ve because the r e inforcing is near the outer fibers where it will resist stress most effec­tively. It a ls o provides a good containment of the masonry.

Surface Bonding

This i s one of the newer techniques , bond i ng glass fiber reinfo rced cement plaster to masonry surfaces. A new ASTM standard has been in1tiated .

Adhesives

The new epoxies, high strength adhesives, have been used in very extensi ve and successful methods afte r many rather clumsy initial attempts. One effective me thod is the forcing of thin epoxy into crevices , providing bond and sealing as well as adding tension capacity or reinforcing.

The newer foaming adhesives demons trate many advan­tages, especially for filling large and irregular cavities .

Grout Intrusions

Intrusions of cement grout into cracks and interstices have been done and is effective in st rengthening open cra c ks .

Ory Pack

Cutting out damaged ma te rial and replacing with low water content, low shrinkage portland cement aggregate mi x, similar to a dental repair method, is a good economical procedure.

Drilling and bolting repairs have been used success­fully for mechanical attachments.

APPENDIX

Verification of Existing MateriaI s

A. Identify structural system or scheme of framing

1. Dverall system

a. Vertical b. Lateral

B. Types of structural elements and frequently , noted typical damage to be observed or to check for in each

1. Element may provide several functions simul­taneously

2. Non-bearing walls

a. Curtain b. In -fi lled c. Frequent failures. Crushing at corners,

joint sliding at top or bottom. diagonal

or X c racks through units, or through joint pattern. horizon tal cracks due to bending. spanning from top to bottom, vertica l c racks due to horizontal span , spalling .

3 . Beari ng walls

a. Loading , eccentric, ar concentric b. Connections , and multip le function c. Frequent failures. Crushing or sp litting

of corne r s . sliding of top or bottom. X cracks through units. or t hrough joint pattern, ho ri zontal cracks due t o ve rt ica l span, ve rtical cracks due to hori zontal span , which may be similar to shrinkage cracks , spalling under concentrated loads, connection or bolt loosen ing.

4. Shear walls

a . Reinfo r ced b . Unreinforced c. In-filled d. Co nne ctions e. Failures as in w·alls above.

5. Piers

a . I ndividua l, as small wa ll b. Portions of wal l s w/ or w/o spandrel re­

straint both ends c. Failures as above

6. Lintel s , spandrels and beams

a. Define function b . Load . ve rtical, perpendicular to face c. End s upport d. Frequent failures . Deflection. with tri­

angular cracked area above. vertical crack at center, cracks in jambs under end sup­ports. sliding on supports, diagonal or X shear crack . at ends or at center.

7 . Arches

a. Not freq uento but many masonry elements functi on as arches

b. Must have adequate abutments for reaction of force polygon

c. Frequent failures. Crushing due to ex­cessive compression. yielding abutment. sliding on abutment. collapse due to ec­centric loads and deflection.

8. Columns

a. Individual b. Pilasters c. Vertical load. concentric d. Eccentric load. actual or virtual e. Frequent failures. Crushing. spalling or

splitting under seat load. base spall due to rocking. bed joint cracks due to bending.

9. Types of testing

a. Sonic b. Percussion c. Electromagnetic d. Coring e. Cutting prisms f. Load testing

C. Deterioration to be noted

1. Thermal cracking

a. Long walls b. At openings c. At intersections

2 . Shrinkage of cementit ious material

3. Expansion of clay products

a . Exposure to moisture b . Restraint

4. Spalling. flaki ng and powdering . due to effl orescence action

a. Coat ed or non-coated b . Moisture source

5. Yielding of supports

a . Foundation differential settlement b . Beam deflections c . Intersecting walls d . Late ral supports e . Pilaste r deflection

6 . Impact

a . Moving equipment b . Opening edges due to traffic c . Vi bratory

7. Corros ion

a. Ancho rs b . Rebar . with expansive action of steel c . Chemicals on cement d . Aluminium trim. etc .

8 . Fire damage

a . Clay units. general ly none b . Concre te units . surface condition c . Mortar . surface d . Effect on fittings and anchors e . Effect of contiguous structural elements f . Oef lection or distortion

O. Empirical co de design prov1s10ns o r industry stan­dards may not cover certain special conditions . Appropriate for "average " conditions . Requires thorough investigati on .

1 . Oeflection and stiffness

a . Code values not precise for true deflec­tion and stiffness

b . If required . as for dynamic design o deter­mine by testo do not assume

:ç. Shear stress

a . Tables not safe for old masonry. should have a test for s hea r

b . Tables do not provide for increase of capacity due t o imposition of compressi ve stress

5 . c . 1-5

3 . Code does no t pro vide specifically for a r ching action of masonry

a . Requi r es development of adequate r eaction b . Cons ider p"ossibility of cyclic loading de­

gradation

4 . Wall or column buckling st r ess reduction

a . Does not provide for variation in end re­straint . Effective h may be 1 or 2X existing c l ear h depending on different e'1d condit i ons

b . Actual sp a n is ge nerall y governed by bending stresses nor"mal to wall face

c. Empirical limit f or bear ing wall of h/ t at 25 not valid

5 . Combined bending and direc t stress

a. Co de does not provide f or eccentric load b . Simple equation gives conservative resu l ts c . May combine by PIA ± Mc/ I f or i nc reas ed .

more reasonable use

6 . Bolt va l ues

a . Low fo r l ow strength masonry and unin­spected work

b . For inspected installation may be higher

Oetai l s of Exec uti on

A. Orawings

1 . Oe tai l s

2 . Procedures

a . These are more impo rta nt than in new con­struction and must be spelled out ca re ­fully

B. Inspect ion

1. Must be close and th or ough

2 . Good work record of j ob conduct. i . e .• com­p lete diary

3 . Photographs are good record o