General notes on modeling using sap ecg v1

ECG

ENGINEERING CONSULTANTS GROUP S.A.

General notes on modeling using SAP

Prepared by Dr. Ahmed Ghallab

Version (1.0) 10-5-2009

Prepared by: Dr. Ahmed Ghallab

1

ECG

ENGINEERING CONSULTANTS GROUP S.A.

General notes on modeling using SAP 1. Before modeling 1.1. Column and shear walls locations and distribution In choosing suitable locations for lateral force resisting structural system, the following aspects should be considered: 1. In case if columns are used to resist the lateral load: 1. For small building (ex. Villa), columns should be distributed in both directions to resist lateral load; If the building area is a square and the columns are rectangular, roughly 50% of columns should be distributed in each directions.

2. In case if shear walls are used to resist the lateral load: 1. Shear walls and cores should be with enough stiffness to resist lateral loads in both directions.

Prepared by: Dr. Ahmed Ghallab

2

ECG

ENGINEERING CONSULTANTS GROUP S.A.

2. For the best torsional resistance, as many of the walls as possible should be located at the periphery of the building. Such an example is shown in the following figure. The walls on each side may be individual cantilevers or they may be coupled to each other.

3. The more gravity load can be routed to the foundation via a structural wall, the less will be the demand for flexural reinforcement in that wall and the more readily can foundations be provided to absorb the overturning moments generated in the wall.

4. In multistory building situated in high seismic risk area, a concentration of the total lateral force resistance in only one or two structural walls is likely to introduce very large forces to the foundation structure, so that special enlarged foundations may be required.

Prepared by: Dr. Ahmed Ghallab

3

ECG

ENGINEERING CONSULTANTS GROUP S.A.

2. Load path Load should be transferred from slab to foundation. Circular path as shown in the following figures is not allowed.

3. Distribution of floor loads: Distribution of floor loads is based on the stiffness of the supporting elements (EI/L) NOT ON THE INERTIA ONLY.

Prepared by: Dr. Ahmed Ghallab

4

ECG

ENGINEERING CONSULTANTS GROUP S.A.

Simplified Load Transfer

To Lines

To Points

To Lines and Points

Transfer of Area Load

Slab System Behavior

D B

Slab T = 200 mm Beam Width, B = 300 mm Beam Depth, D a) 300 mm b) 500 mm c) 1000 mm

5.0 m

Moment Distribution in Slabs Only Effect of Beam Size on Moment Distribution

a) Beam Depth = 300 mm

b) Beam Depth = 500 mm

c) Beam Depth = 1000 mm

moment distribution (M 2-2) Effect of beam stiffness on slab moment distribution Prepared by: Dr. Ahmed Ghallab

5

ECG

ENGINEERING CONSULTANTS GROUP S.A.

4. Analysis methods Linear-Inelastic

Action

Action

Linear-Elastic

Deformation

Action

Action

Deformation

Nonlinear-Inelastic

Nonlinear-Elastic

Deformation

Deformation

5. Materials a. Young's Modulus (Ec ): for concrete elements should be calculated from the following equations: Ec = 4400 Fcu

OR

Ec = 14000 Fcu

N / mm 2 kg / cm 2

Where Where

Fcu

Fcu in

in

N / mm 2

kg / cm 2

6. Frame elements a) Sections:

Frame sections are defined as follows: •

R-sec:

Prepared by: Dr. Ahmed Ghallab

Define using b, t

6

ECG

ENGINEERING CONSULTANTS GROUP S.A.

•

Define as T-sec taking flange width = 0.5 Flange width of

T- sec

T-sec ; minimum of (B = 8ts + b OR B=L/10) •

Define as T-sec taking flange width = 0.5 Flange width of

L- sec

L-sec ; minimum of (B = 3ts + b OR B= L/20)

Note: for L-sec and T-sec the section can be defined as R-sec and multiply the inertia

around the main axis (3) ; I3-3- by the ratio: (

I3- 3 (T - sec or L - sec) ) I3-3 (R - sec)

For T-sec this ratio can be taken as (2) For L-sec this ratio can be taken as (1.5) Prepared by: Dr. Ahmed Ghallab

7

ECG

ENGINEERING CONSULTANTS GROUP S.A.

b) Local axes

The local axis 3 should be parallel to the width (b) in all cases as shown in the above figure. c) Flexural rigidity (Ec I)

•

under vertical loads or wind load

When calculating the internal forces in the building flexural rigidity of the members can be calculated by one of the following methods (ECP 203-2007, 6-3-1-5): 1. Flexural rigidity of all members in the building can be calculated based on the full member dimensions and neglecting steel bars; Ec Ig ; For case of R-sec Ig = bt3/12 2. effect of cracks can be taken into consideration by taking inertia of columns =Ec Ig and inertia of beams = 0.5 Ec Ig Note: only one method should be used in the whole building.

Table (1): (Recommended values for design and to consider effect of cracking in deflection calculation) for GRAVITY and WIND loads

member

ECP

Columns

Ig

Shear walls

Ig

beams

0.5 Ig*

Flat plates and flat slabs

0.5 Ig

ACI-318 (2008)

UBC (1997)

and solid slabs Prestressed slab

Ig

* For T-sec use 2x Ig, For L-sec use 1.5 x Ig (as discussed before)

Prepared by: Dr. Ahmed Ghallab

8

ECG

ENGINEERING CONSULTANTS GROUP S.A.

•

Flexural rigidity under seismic load

To calculate the lateral drift and internal forces due to seismic load, the inertia of the members are taken as follows (to take the effect of the cracking): Table (2): Stiffness modifiers for SEISMIC load case

member

ECP

ACI-318 (2008)

UBC (1997)

Columns

0.7 Ig

0.7 Ig

0.7 Ig

0.7 Ig (uncracked)

0.7 Ig (uncracked)

0.7 Ig (uncracked)

0.35Ig (cracked)**

0.35 Ig (cracked)

0.35 Ig (cracked)

beams

0.35 Ig*

0.35 Ig

0.35 Ig

Flat plates and flat

0.25 Ig

0.25 Ig

0.25 Ig

Shear walls

Prestressed slab 0.25 Ig * For T-sec use 2 Ig, For L-sec use 1.5 Ig (as discussed before) ** (Assume wall section is uncracked (Iwall = 0.7 Ig ) as a first assumption after analysis check if it is cracked or not. If the moment is bigger than the cracking moment repeat analysis using Iwall = 0.35 Ig NOTE: These factors are for seismic force only and not for wind load. For wind load

values in Table (1).

-------------------------------d) Torsion rigidity

•

Case of equilibrium torsion

In this case redistribution of torsion is NOT allowed and the member should be designed based on the acting torsion.

•

Case of compatibility torsion

In this case redistribution of torsion is allowed and the torsion rigidity of the member can be considered equal to zero in analysis but additional rft, enough to resist torsion equals to cracking torsion moment, should be added (for small value of torsion moment stirrups used to resist shear could be enough)

Prepared by: Dr. Ahmed Ghallab

9

ECG

ENGINEERING CONSULTANTS GROUP S.A.

Prepared by: Dr. Ahmed Ghallab

10

ECG

ENGINEERING CONSULTANTS GROUP S.A.

Recommended value for torsion constant in Sap and Etabs : Compatibility torsion= 0.01 Equilibrium torsion = 0.2

Prepared by: Dr. Ahmed Ghallab

(assumed cracked)

11

ECG

ENGINEERING CONSULTANTS GROUP S.A.

7. Shell elements a) Shell element shape

b) Type Thin shell element generally used to model flat slab Thick shell element generally used to model raft slab c) Meshing The term ‘mesh’ is used to describe the sub-division of surface members into elements (see Figure 4), with a finer mesh giving more accurate results. The engineer has to assess how fine the mesh should be; a coarse mesh may not give an accurate representation of the forces, especially in locations where the stresses change quickly in a short space e.g. at supports, near openings or under point loads. This is because there are insufficient nodes and the results are based on interpolations between the nodes. However, a very fine mesh will take an excessive time to compute, and is subject to the law of diminishing returns

Prepared by: Dr. Ahmed Ghallab

12

ECG

ENGINEERING CONSULTANTS GROUP S.A.

Fig. ( ) Bending moments: accuracy of results compared with mesh size d) Shells numbers: For lateral load analysis (to design columns, walls and check drift) use a coarse mesh, for example, ( 2 x 2 units) For design of slabs and beams, use moderate mesh; the least of:

•

Minimum number (4x 4 units).

•

Maximum width (1x1 m.)

Prepared by: Dr. Ahmed Ghallab

13

ECG

ENGINEERING CONSULTANTS GROUP S.A.

e) Local axes Local axes of shell elements in the same plan should be in the same direction

8. Structureal Modeling a) Under vertical load: a) It is better to represent the whole structure as 3-d model. However, in some cases (simple structures) 2- d model can be faster.

Prepared by: Dr. Ahmed Ghallab

14

ECG

ENGINEERING CONSULTANTS GROUP S.A.

b)

Each floor can be represent as 2-d model (in plan) but columns should be modeled above (if any) and bottom the floor.

c)

In some programs (ex. Safe) supports (ex. Columns) can be modeled as rotational springs with defined stiffness of rigidity (column stiffness). For a pin-ended column the stiffness can be taken as K = 3EI/l and for a fully fixed column K = 4EI/L

Prepared by: Dr. Ahmed Ghallab

15

ECG

ENGINEERING CONSULTANTS GROUP S.A.

b) Under horizontal load: d) Under earthquake load (using response spectrum response) Building should be modeled in the 3-D e) Under earthquake load (using equivalent static method) or wind load Building can be modeled in the 3-D or 2-D dimensions.

9. Revised your model a) Use body constraints to model a column (or shear wall ) carries several beams (as shown below)

Prepared by: Dr. Ahmed Ghallab

16

ECG

ENGINEERING CONSULTANTS GROUP S.A.

In this case column (or shear wall) is modeled using frame element at its center line (point) and the points on its edges are constrained to the center point as follows

Prepared by: Dr. Ahmed Ghallab

17

ECG

ENGINEERING CONSULTANTS GROUP S.A.

Prepared by: Dr. Ahmed Ghallab

18

ECG

ENGINEERING CONSULTANTS GROUP S.A.

Alternatively, a stiff beam with small area section can be used as below:

.

b) Case of 2-D modeling 1- Column can be represented by a one support or several supports (based on the dimensions of column relative to the shell element dimension). In the later, the column load is the algebraic summation of the supports reactions

Prepared by: Dr. Ahmed Ghallab

19

ECG

ENGINEERING CONSULTANTS GROUP S.A.

2- Use body constraints as discussed before (Better solution) c) Moment release Moments at the ends of beams can be neglected (released) but additional top reinforcement at the edge connections enough to resist moment equals to (wL2/24) should be added. (if beam length is less than 8 m). if beam is longer than 8 m, connecting moment will be higher and may result in considerable cracking.

For beam, release moment around axis3

Prepared by: Dr. Ahmed Ghallab

20

ECG

ENGINEERING CONSULTANTS GROUP S.A.

d) Moment release (Drop in slab; bathroom slab) In case of drop in slab, discontinuity can be modeled by release moment in the studied direction

Prepared by: Dr. Ahmed Ghallab

21

ECG

ENGINEERING CONSULTANTS GROUP S.A.

Prepared by: Dr. Ahmed Ghallab

22

ECG

ENGINEERING CONSULTANTS GROUP S.A.

Discontinuity in x and y directions

10. Cases of loadings Each case of loading should be represented separately; 1. Dead Load (D) 2. Floor cover (FC) 3. wall load (WALL) 4. Live load (L) 5. False ceiling (FA) 6. Mechanical loading (M) 7. earthquake (E) 8. wind (W) 9. Temperature (T)

Prepared by: Dr. Ahmed Ghallab

etc.,

23

ECG

ENGINEERING CONSULTANTS GROUP S.A.

11. Cases of combinations

Prepared by: Dr. Ahmed Ghallab

24

ECG

ENGINEERING CONSULTANTS GROUP S.A.

Prepared by: Dr. Ahmed Ghallab

25

ECG

ENGINEERING CONSULTANTS GROUP S.A.

Prepared by: Dr. Ahmed Ghallab

26

ECG

ENGINEERING CONSULTANTS GROUP S.A.

UBC-1997 COMBINATION

Prepared by: Dr. Ahmed Ghallab

27

ECG

ENGINEERING CONSULTANTS GROUP S.A.

ρ = 1 in Seismic Zones 1 and 2

Prepared by: Dr. Ahmed Ghallab

28

ECG

ENGINEERING CONSULTANTS GROUP S.A.

12. Ultimate limit state design Twisting moments

Prepared by: Dr. Ahmed Ghallab

29

ECG

ENGINEERING CONSULTANTS GROUP S.A.

NOTE: generally mxy can be assumed = 0.2 maximum span moment in each direction (according to the limit of total moment).hence M (+ve) design:= 1.2 Mult.

13. Deflection

Prepared by: Dr. Ahmed Ghallab

30

ECG

ENGINEERING CONSULTANTS GROUP S.A.

a) Minimum thickness of one way solid slab

b) Minimum thickness of two way solid slab

Prepared by: Dr. Ahmed Ghallab

31

ECG

ENGINEERING CONSULTANTS GROUP S.A.

c) (4-3-1-3) Deflection calculation can be neglected if:

Prepared by: Dr. Ahmed Ghallab

32

ECG

ENGINEERING CONSULTANTS GROUP S.A.

If the above values are not satisfied deflection should be calculated as follows

Prepared by: Dr. Ahmed Ghallab

33

ECG

ENGINEERING CONSULTANTS GROUP S.A.

14. Deflection calculations: Deflection due to each case of loading should be calculated separately using any of the following methods then compared with the code limits.

c.1. Using manual method

Prepared by: Dr. Ahmed Ghallab

34

ECG

ENGINEERING CONSULTANTS GROUP S.A.

c.2. Using sap model In sap model deflection can be calculated from the results then total long term deflection can be calculated from the following relation

∆total (long term) = ∆live +(1+α )(∆ ∆ dead + ∆ cover + ∆ wall) The above equation is based on stiffness modifier factors given in table (1). If the stiffness of slabs is taken as the gross value, ∆total sould be increased to take effect of cracking.

Prepared by: Dr. Ahmed Ghallab

35

ECG

ENGINEERING CONSULTANTS GROUP S.A.

--------------------

15. Springs under footing or raft To represent the contact between soil and footing, joint sprting or area spring can be used. Stiffness of spring can be calculated as follows:

a) 7.1

Joint spring: K = (100-120) qall * element area (m2)

b) 7.2

Area spring (RECOMMENDED) K= (100-120) * qall

Where qall = allowable soil bearing stress under the footing (ton/m2) Prepared by: Dr. Ahmed Ghallab

36

ECG

ENGINEERING CONSULTANTS GROUP S.A.

16. Pile stiffness (ref. Design of Pile Foundations Axial Stiffness. The axial pile stiffness is expressed as:

__

where b33 = axial pile stiffness C33 = constant which accounts for the interaction between the soil and the pile A = cross-sectional area of the pile E = modulus of elasticity of the pile L = length of the pile The term AE/L is the elastic stiffness of the pile acting as a short column with no soil present. The coefficient (C33) accounts for the stiffness of the soil-pile system.

For design purposes, C33 for a compression pile ranges between 1.0 and 2.0 although values as low as 0.1 and as high as 3.0 have been noted in the literature.

There appears to be a relationship between C33 and pile length. Longer piles tend to have higher values of C33 than shorter piles.

C33 for tension piles in sand can be taken as one half of the value used for compression piles. For tension piles in clay use 75 to 80 percent of the value of C33 for compression piles.

(2) Long-term loading, cyclic loading, pile group effects, and pile batter can affect C33 .

In sand, long-term loading has little effect on the value of C33 ; however consolidation in clay due to long-term loading can reduce C33 . At present, the effect of cyclic loading on C33 is neglected.

For design purposes, if piles are driven to refusal in sand or to a hard layer, there is no change in the value of C33 for pile groups; however, C33 may be reduced for groups of friction piles.

Prepared by: Dr. Ahmed Ghallab

37

ECG

ENGINEERING CONSULTANTS GROUP S.A.

17. Reinforcement details

18. REFERENC O Brooker " How to design reinforced concrete slab using Finite element analysis" The Concrete Centre, May 2006

Prepared by: Dr. Ahmed Ghallab

38

ECG

ENGINEERING CONSULTANTS GROUP S.A.

Appendices Appendix A Typical damping ratio

Prepared by: Dr. Ahmed Ghallab

39