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EERI – PAVIA Workshop

Reinforced Concrete Frames

(Predating seismic codes)

Ing. Luis G MejíaMedellín - Colombia


1.4 General Information

Summary:This is a typical multi- family housing constructionfound in urban areas of Colombia that predates seismiccodes. At the present time, poor and middle closspeople occupy buildings of this type. This type ofconstruction is rather vulnerable to seismic effects dueto a limited amount of transverse reinforcement (ties);This is especially true for columns. This structuralsystem is very flexible when subjected to lateral seismicloads. The quality of materials and workmanship istypically rather poor. In many cases, buildings of thistype are constructed on a very steep terrain; Soilcondition is often rather poor.

Nowadays poor people build similar buildings withoutany seismic characteristics


4. Structural Features

4.1 Lateral Load- Resisting System:

This type of building frame does not have anyearthquake- resisting features.

4.2 Gravity Load- Bearing Structure:

Like in a regular frame structure, vertical loads aretaken by the joists, which are supported by thegirders; the girders transfer the load to the columns.Often the slabs are constructed using tile blocks andconcrete joists and girders. Beams and columns areconstructed in a manner typical for reinforced concretestructures


4.3 Type of Structural System

Moment resisting frame

13 Designed for gravity loads only (predating seismic codes i.e. no seismic features) X

14 Designed with seismic features (various ages)

15 Frame with unreinforced masonry infill walls

16 Flat slab structure

17 Precast frame structure

18 Frame with concrete shear walls- dual system

Shear wall structure

19 walls cast in- situ

20 Precast wall panel structure

Note: Un reinforced masonry infill walls were and arenowadays used. Because the buildings are very slenderThese is a big possibility of danger with these walls withbad consequences for inhabitants and pedestrians.


Type Description (X)

Shallow foundation

Wall or column embedded in soil, without footing

Rubble stone, fieldstone isolated footing

Rubble stone, fieldstone strip footing

Reinforced concrete isolated footing X

Reinforced concrete strip footing

Mat foundation

No foundation

4.4 Type of Foundation

Deep foundation Reinforced concrete bearing piles X

Reinforced concrete skin friction piles x

Steel bearing piles

Wood piles

Steel skin friction piles

Cast in place concrete piers

Caissons

Notes: Regularly shallow foundations are used. Sometimes,specially with soft soils (SPT 5- 10), Reinforced Concrete Piles(friction, bearing or frictions plus bearing) were used. Tie beamsweren’t used


4.5 Type of Floor System

Material Description of floor/roof system Floor Roof

Masonry Vaulted

Composite masonry and concrete joist X

Structural

concrete

Cast in place solid slabs

Cast in place waffle slabs

Cast in place flat slabs

Precast joist system

Precast planks

Precast beams with concrete topping

Postensioned slabs

Steel Composite steel deck with concrete slab


4.5 Type of Roof System

Material Description of floor/roof system Floor Roof

Timber Rammed earth with ballast and concrete or plaster finishing

Wood planks or beams with ballast and concrete or plaster finishing

Thatched roof supported on wood purlins

Wood shingle roof

Wood planks or beams that support clay tiles X

Wood planks or beams that support slate, metal, asbestos- cement or plastic corrugated sheets or tiles

Other


5.3 Seismic Vulnerability Rating:

High Vulnerability=

Very Poor Seismic

PerformanceA B

MediumVulnerability

C D E

Low Vulnerability=Excellent Seismic Performance

F

Seismic Vulnerability Class

| - - |


6. Earthquake Damage Patterns

YearEarthquakeEpicenter

Richter magnitude

(M)

Maximum Intensity

Number of buildings of this type that completely

or partially collapsed (X)

FEW SOME MOST (*)

November23/79

4.8N, 76.2W, depth: 108 km(Mistrató)

6.7 Ms VIII MM(Manizales)

x

March31/83

2.46N, 76.69W, depth: 22 km (Popayán)

5.5 Mb IX MM(Popayán)

x

February8/95

4.1N, 76.62W, depth: 73 km (Pereira)

6.4 Mw VIII MM(Pereira)

x

January25/99

4.46N, 75.72W, depth: 17 km (Armenia)

6.0 Ms IX MM(Armenia)

x

Late seismic events

Note: The damages are big whenever the population, thedimension of buildings grow and whenever theearthquake is more superficial


Photographies Illustrating Typical Earthquake Damage


Figures 6

Importance of a good construction practiceand its impact on seismic performance ofbuildings is obvious from the enclosedphotos.


Detail ADetail B

Collapsed building

Note: We will see the details A and B in the next two diapositives.

Building 1:


Splices in joints

Detail A

Detail B


Anchorage of beam reinforcement in column cover (not inside the joint)

Fig 6: It should be remembered that onebuilding will behave as constructed andnot as designed.

Detail B


(This was the first story)

Very poor longitudinal and transverse reinforcement

Building 2: Collapsed Building

EERI – PAVIA WorkshopEERI – PAVIA Workshop

Building A Building B

Detail A

Comments: Both buildings A and B are of orthogonal plan and similarconstruction, however only one suffered a severe damage. Note thatthe walls are not anchored to the floor/roof diaphragms.

Building 3: Collapsed Building


Detail A: Too widely spaced stirrups

EERI – PAVIA WorkshopEERI – PAVIA Workshop

Secondstory

Collapsed Building: This used to be a 3- storybuilding. Note the total lateral displacement(drift) between the 2nd and the 3d floor.

Building 4:

Detail A


Column detail: Widely spaced stirrups and poor quality of aggregate

(This was the first story)

(Second story)

Detail A


Too slender columns: excessive P- δ effects

Slender columns

Building 5: Damaged building


A building under construction with completely inadequate column splices. Note a very poor transverse reinforcement.

Building 6:


Comments: Reinforcement withoutcover or with insufficient cover isineffective or only partially effective.

Insufficient coverBuilding 7: Bad construction details


10. Seismic StrengtheningTechnologies


10. Seismic Strengthening Technologies

Type of intervention

Structural Element

Description of seismic strengthening provision

Increase in the construction cost, see Note 1Select one (X)

Likelihood of enhancing seismic stability, see Note 2 Select one (X)

High (>5%)

Medium (2- 5%)

Low (<2%)

High Medium Low

Retrofit (strengthening)

Beams and columns

See fig. 7A,7B and 7C Photos x x

Notes:1. The procedures illustrated below are not complex in design orconstruction, however they require good planning and a perfectcoordination between the owner, the designer and the builder.


FIGURE 7: Illustration of Seismic

Strengthening Techniques


Figure 7A

Strengthening technique 1: Only stirrups are added to avoid a fragile failure


Figure 7B

Strengthening technique 1: New longitudinal and transversal reinforcement is added in columns


Figure 7C

Strengthening technique 3: New longitudinal and transversal reinforcement is added in beams and columns


Comments to figure 7A, 7B and 7C :- It is important to note that there are different

grades of difficulty with respect to theeffectiveness among the techniques # 1, 2 and 3shown on Figures 7A, 7B and 7C respectively.For example, the technique #1 is considerablysimpler in terms of construction as comparedwith the technique 3, however on the other handit is much less effective as compared to thetechnique # 3.

- In addition to the above techniques, newseismic strengthening techniques using carbon(glass) fibers are also in use, although for normalbuildings these procedures are very expensive.


Typical Strengthening Procedures

EERI – PAVIA WorkshopExtraction of concrete cores

Material investigation for example:


Step 1: Anchorage of the new longitudinalreinforcement into the foundation

Column intentionally

Rougned


Step 2 : New added longitudinal and transversal reinforcement


Step 3: Epoxy injection of the cracks


In some cases, new stirrups areadded to reinforce a beam-column joint. In this case, theexisting concrete in the joint areamust be carefully demolished.

SEISMIC STRENGTHENING TECHNIQUES - DETAILS

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