Why Long-Term Deformation should be considered?

November 18, 2021
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OVERVIEW

In high-rise buildings, the axial deformations of columns cannot be ignored, and special considerations are required for design and construction. A vertical member undergoes both elastic deformation and deformation due to creep and shrinkage. The elastic deformation takes place instantaneously due to dead loads and live loads applied to the structure, while deformation due to creep and shrinkage occurs over many years. Most of the vertical deformations in a high-rise building, however, take place during its construction.

Due to the difference in axial stiffness and load distribution areas on vertical members, differential shortening inevitably develops. If this differential shortening in vertical members, which takes place during and after the construction, is not considered in analyses of high-rise buildings, structural safety will be compromised due to additional stresses in the horizontal members and, subsequently, in the vertical members. The structural safety problem is also magnified when incorporating serviceability issues such as the curtain wall function, floor unevenness, excess stress in piping, etc. As such, total displacements of vertical members must be calculated at the design stage. Comparatively reasonable and accurate results can be predicted when construction stage analysis is carried out reflecting the creep and shrinkage behavior of concrete.

Conventional structural analysis has the assumption that all structural loads are instantaneously applied to the entire completed structure. However, since most buildings are constructed by one story or several floor units at a time, or even if it is the same story, the construction sequence and loading sequence may be different depending on the construction plan. Therefore, the actual structural behavior can be significantly different from the conventional analytical behavior based on the above assumption.

Vertical members (columns and walls) in high-rise reinforced concrete buildings not only exhibit elastic shortening, but also have shrinkage and creep effects that develop from long-term compressive loading. In lower stories of a building, additional stresses in girders become very large due to differential shortening and undergo significant redistribution of the member forces.

In order to analytically solve the problem described above, the construction stage analysis function of midas Gen considers shrinkage and creep during construction stages to simulate the construction process of a high-rise building. Also, with input variables, such as the strength of concrete, construction duration of building components, casting condition, ambient condition, etc., the elastic shortening, shrinkage and creep of vertical members can be estimated and are reflected in the analysis. Change in strength gain based on the maturity of concrete members is also reflected in the calculation of modulus of elasticity at various construction stages.

In the following example, construction stage analysis considers the creep and shrinkage effects of a 40-story building consisting of an exterior concrete frame and interior shear walls, as shown in figure 1. The displacements of vertical members and the girder member forces are compared and evaluated with the results from conventional analysis (analysis in which construction stages are not considered.).

 

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Structural System


The structural system, as shown in figure 1, is a 40-story building constructed with core walls and perimeter RC columns & RC girders. In order to evaluate the influence that gravity has on the displacements of a vertical member, and the member forces of horizontal members, the member sections are selected according to the dead load and the live load. The typical plan view of a story is shown in figure 2.

3D Analysis Model

Figure 1. 3D Analysis Model

 

 

Typical Story Plan View

Figure 2. Typical Story Plan View

 

 

 

Design Loads


 

Type of Loads

Symbols

Loads

Frame Self Weight

SW

-

Slab Self Weight

Dc

360kgf/m2

Finishing and Masonry Loads

Ds

160kgf/m2

Live Loads

LL

250kgf/m2

 

 

 

 

 

Construction Sequence


 

Stage

Element Group

Stage Load

Duration (days)

Total Construction Period (days)

Description

#CS1

1st Fl. Core

 

SW/Dc

5

5

 

#CS2

2nd Fl. Core

 

SW/Dc

5

10

 

#CS3

3rd Fl. Core

 

SW/Dc

5

15

 

#CS4

4th Fl. Core

1st Fl. Frame

SW/Dc

5

20

 

#CS5

5th Fl. Core

2 Fl. Frame

SW/Dc

5

25

 

#CS6

6th Fl. Core

3 Fl. Frame

SW/Dc

5

30

 

#CS7

7th Fl. Core

4 Fl. Frame

SW/Dc

5

35

 

 

#CS21

21st Fl. Core

18 Fl. Frame

SW/Dc/Dd

5

105

1st Fl. Int. Finishing

#CS22

22nd Fl. Core

19 Fl. Frame

SW/Dc/Dd

5

110

2nd Fl. Int. Finishing

#CS40

40th Fl. Core

37 Fl. Frame

SW/Dc/Dd

5

200

20th Fl. Int. Finishing

#CS41

 

38 Fl. Frame

SW/Dc/Dd

5

205

21st Fl. Int. Finishing

#CS42

 

39 Fl. Frame

SW/Dc/Dd

5

210

22nd Fl. Int. Finishing

#CS43

 

40 Fl. Frame

SW/Dc/Dd

5

215

23rd Fl. Int. Finishing

#CS44

 

 

Dd

85

300

24~40 Fl. Int. Finishing

#CS45

 

 

 

90

390

 

#CS46

 

 

LL

260

650

Live Loads

 

 

Sequence of Construction Stages

Figure 3. Sequence of Construction Stages

 

 

 

 

Material Properties


In order to define the properties of concrete shrinkage and creep, the ACI standard is used, and the concrete material properties are shown in this table.

Column

Comp.

Strength

(kgf/cm2)

Humidity (%)

Slump

(cm)

Aggregate (%)

Air

(%)

Cement

(kg/m3)

V/S

Ratio

(mm)

Code

C1

(1~5th)

500

55

12

60

4.5

450

300

ACI

C1

(6~10th)

500

55

12

60

4.5

450

275

ACI

C1

(11~15th)

400

55

12

60

4.5

380

250

ACI

C1

(16~20th)

400

55

12

60

4.5

380

225

ACI

C1

(21~25th)

350

55

12

60

4.5

350

200

ACI

C1

(26~30th)

350

55

12

60

4.5

350

175

ACI

C1

(31~35th)

300

55

12

60

4.5

320

150

ACI

C1

(36~40th)

300

55

12

60

4.5

320

125

ACI

C2

(1~10th)

500

55

12

60

4.5

450

225

ACI

C2

(11~20th)

400

55

12

60

4.5

380

200

ACI

C2

(21~30th)

350

55

12

60

4.5

350

175

ACI

C2

(31~40th)

300

55

12

60

4.5

320

150

ACI

Girder G1

270

55

12

60

4.5

302

127

ACI

Girder G2

270

55

12

60

4.5

302

100

ACI

Link beam

270

55

12

60

4.5

302

127.3

ACI

Wall

(1~10th)

500

55

12

60

4.5

450

266

ACI

Wall

(11~20th)

400

55

12

60

4.5

380

266

ACI

Wall

(21~30th)

350

55

12

60

4.5

350

226

ACI

Wall

(31~40th)

300

55

12

60

4.5

320

226

ACI

 

The concrete creeps coefficient data and graph, as well as the shrinkage strain data and graph, as per the ACI standard, are shown in figure 4 and figure 5, respectively.

 

Concrete Creep Coefficient Data & Graph by ACI Code

Figure 4. Concrete Creep Coefficient Data & Graph by ACI Code

 

 

Shrinkage Strain Data & Graph by ACI Code

Figure 5. Shrinkage Strain Data & Graph by ACI Code

 

 

 

 

 

 

 

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About Author
Yeong-il Seo | Principal Structural Engineer

Young-il has over 13+ years of experience in building design, especially high-rise buildings with column reduction analysis, plant structures, pushover analysis, health monitoring, and vibration control projects. Since 2016, he is planning and providing technical supports for midas building products such as midas Gen, nGen, and Design+.

E-BOOK Construction Stage
Analysis Reflecting
Long-Term Deformation

This material explains construction stage analysis and practical considerations
for structure design and construction.