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Reinforced Concrete Frames
(Predating seismic codes)
Ing. Luis G MejíaMedellín - Colombia
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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
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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
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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.
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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
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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
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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
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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
| - - |
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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
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Photographies Illustrating Typical Earthquake Damage
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Figures 6
Importance of a good construction practiceand its impact on seismic performance ofbuildings is obvious from the enclosedphotos.
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Detail ADetail B
Collapsed building
Note: We will see the details A and B in the next two diapositives.
Building 1:
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Splices in joints
Detail A
Detail B
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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
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(This was the first story)
Very poor longitudinal and transverse reinforcement
Building 2: Collapsed Building
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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
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Detail A: Too widely spaced stirrups
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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
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Column detail: Widely spaced stirrups and poor quality of aggregate
(This was the first story)
(Second story)
Detail A
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Too slender columns: excessive P- δ effects
Slender columns
Building 5: Damaged building
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A building under construction with completely inadequate column splices. Note a very poor transverse reinforcement.
Building 6:
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Comments: Reinforcement withoutcover or with insufficient cover isineffective or only partially effective.
Insufficient coverBuilding 7: Bad construction details
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10. Seismic StrengtheningTechnologies
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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.
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FIGURE 7: Illustration of Seismic
Strengthening Techniques
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Figure 7A
Strengthening technique 1: Only stirrups are added to avoid a fragile failure
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Figure 7B
Strengthening technique 1: New longitudinal and transversal reinforcement is added in columns
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Figure 7C
Strengthening technique 3: New longitudinal and transversal reinforcement is added in beams and columns
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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.
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Typical Strengthening Procedures
EERI – PAVIA WorkshopExtraction of concrete cores
Material investigation for example:
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Step 1: Anchorage of the new longitudinalreinforcement into the foundation
Column intentionally
Rougned
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Step 2 : New added longitudinal and transversal reinforcement
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Step 3: Epoxy injection of the cracks
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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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