Minimum reinforcement ratio in piles

1-Introduction :

The Piles are a structural element that can be wood, steel sections, precast concrete, cast-in-place concrete, and composite type piles and etc.

The main function of the piles include :

1- Transferring load from the superstructure through weak compressible strata onto stiffer soils or onto rock.

2-Resisting uplift force when used to support tall structures or basement below the groundwater table.

3- Control the settlement.

4- Used in marine structures to resist the lateral loads from the impact of berthing ships and from waves.

2- Type of piles based on function.

Classification of piles with respect to functional behaviour as follows :

a- End-bearing pile: This type of pile derives most of its capacity from a bearing stratum on which the tip bears.

b- Friction pile: This type of pile derives its resistance primarily from friction or adhesion along the length of the pile. They are commonly used where a bearing stratum is too deep to be usable. A pile that resists tension does so by friction and would be considered a friction pile.

c- Combined end-bearing and friction pile: This type of pile derives its resistance from a combination of end bearing and friction

d- Batter pile: this is a pile that driven at an angle with respect to vertical to resist horizontal force.

e-Mirco pile.

2- Pile reinforcement detail :

Pile reinforcement detail is one of the issues that are unclear, and there is conflicting information in ACI 318, ASCE 7 and IBC foundations, but now ACI 318-19 eliminated that conflict. Below table explains the minimum reinforcement requirements.

References.

1.   Building code requirements for structural concrete (ACI 318-19) and commentary.

2.   International building code 2015

3.   Foundation analysis and design.

Introduction to Diaphragm design.

1-Diaphragms:

Diaphragms is a horizontal or sloped system acting to transmit lateral forces to vertical elements of the lateral force-resisting system (LFRS). Generally provided by the floor and roof systems of the building; sometimes, however, horizontal bracing systems independent of the roof or floor structure serve as diaphragms.

There are many types of materials and systems for use as floor and roof diaphragms.

Such as; Concrete slabs, Precast concrete floor planks with concrete topping, Metal decking with concrete fill, Roof sheathing .. etc.

2- Diaphragms roles for the structure, the main purpose of the diaphragm is to distribute lateral forces to the elements of the LFRS also it is doing the following:

1- Resist gravity loads.

2-Provide lateral Support to vertical elements.

3-Transfer lateral inertial forces to vertical elements of the seismic force-resisting system.

4- Transfer forces through the diaphragm.

The Largest diaphragm transfer forces should be anticipated at offsets or discontinuities of the vertical elements of the seismic-force-resisting system such as.

(a) Setback in the building profile

(b) Podium level at grade.

3- Diaphragms components:

A-Chord:

Is assumed to resist all the flexural tension from the diaphragm in-plane bending moment resulting from the lateral load. In case the edge beam is not existed the slab will act as a deep beam to resist flexural tension force and the chord tension reinforcement to be placed within h/4 of the tension face, where h is the diaphragm width in the direction of the analysis (section 12.5.2.3 of ACI code).

B-Collector:

Also called drag struts or ties, are diaphragm framing members that collect diaphragm shear forces from laterally unsupported areas to vertical resisting elements. The collector width can be fit within the shear wall width but in some cases, it has to be wider.

SEAOC 2005 recommends the collector effective width beff to not exceed the wall width plus a width on either side of the wall equal to half the contact length between the diaphragm and the wall.

4- Behavior of Diaphragms

The behavior of a diaphragm can be as beam that is supported springing which represents the lateral stiffness of lateral resisting elements. The floor or roof system acts as the beam web which resists the design shear force the chords behave as flange elements resisting the axial tension or compression resulting from flexural behavior.

5-Type of diaphragms:

Diaphragms are typically classified into three categories: rigid, flexible and semi-rigid

a- Rigid diaphragms 

ASCE7-10 ( section 12.3.1.2) permits the assumption of rigid if the diaphragms aspect ratio is 3 or less for seismic and 2 or less for wind load ( section 27.5.4 of ASCE7-10) if the structure has no significant horizontal irregularities. The seismic story shear is to be distributed to the vertical elements of the LFRS based on the relative lateral stiffness of those elements.

b-Flexible:

Diaphragms is permitted to be flexible when the computed maximum in-plane deflection of the diaphragm under lateral load is more than two times the average story drift of adjoining vertical elements of the seismic force-resisting system of the associated story under equivalent tributary lateral load as shown in the above Fig. 12.3-1.

The seismic story shear is to be distributed to the vertical elements of the LFRS based on the tributary area. And the diaphragm deflection is significantly high compare to LFRS

c- Semi-rigid diaphragms simulate actual in-plane stiffness properties and behavior. it should be modeled when significant in-plane deformation does occur, or when required by code.

6- Code requirements:

The seismic design of the diaphragm is required for all buildings in SDC B to F.

ASCE7-10 12.10 required diaphragms to be designed for the internal forces determined as the maximum of :

a-    

b-    

The detailing of diaphragms is generally independent of the type of LFRS for building, therefore, the R-value does not appear in the upper and lower limit equation.

Section 18.12 of ACI 318-14 shall only apply for diaphragm design in building assigned to SDC D to F only. while chapter 12 of ACI318-14 provisions shall be applied and for buildings assigned to SDC B and C.

7- Diaphragm modeling and analysis approaches

The internal force in diaphragm can be calculated from hand calculation till complex computer analysis depending on the building's irregularity.

The following methods can be used for analysis:

1-    Equivalent beam model.

2-    Equivalent beam on spring model.

3-    Corrected equivalent beam model.

4-    Strut and tie model.

5-    Finite element model.

8-How to obtain the diaphragm in ETAB.

The diaphragm forces can be obtained through sections cut as illustrated in the following link:  https://wiki.csiamerica.com/display/etabs/Diaphragm+forces

Bear in mind the Stiffness modifiers for RC diaphragms commonly fall in the range of 0.15 to 0.50 when analyzing the building for design-level earthquake demands (Nakaki, 2000).

References.

1.   Building code requirements for structural concrete (ACI 318-14) and commentary.

2.   Minimum design loads for buildings and other structures (ASCE 7-10)

3.   NEHRP Seismic Design Technical Brief No. 3.

Structural engineer Commons mistakes when using Saudi building code.

Previously, Structural engineers in Saudi Arabia use British or American codes based on their knowledge background because there was no clear guideline or code for them to use in designing . Moreover, Uniform building code provided limited information about seismic zone for few cities in Saudi Arabia .

In 2007 Saudi Building code (SBC) was published and since then we have clear code that satisfy our requirements.Unfortunately, we found out there are three common mistakes, the structural engineers always do when using the Saudi building code.

1-    Mix of codes requirements :

Recently, structural engineers in KSA are using IBC 2012 code /ASCE 7-10 standards along with wind and seismic map provided in SBC code; ignoring the fact of SBC is based on (IBC 2003) code/ ASCE7-02. Which lead to underestimate wind design load, and in some cases it may cause a building failure.

ASCE 7-10 contains significant changes from ASCE 7-02 in the areas of seismic design, wind design, and more.

In ASCE7-10 the wind speed values represent “Ultimate” wind speeds , therefore, Strength design level wind speeds replaces the ASD level wind speeds. But in SBC the wind speed value represent “service “wind speed.

This is the reason the load factor is 1.0 for wind instead in ASCE7-10 While the load factor is 1.6 in SBC code .

Load combinations as per (ASCE 7-10) as follows :

Load combinations as per Saudi building code (SBC) as follows

Therefore, in order to use ASCE7-10 you , the wind speed should be modified .

Fortunately, ASCE7-10 provide a table in the commentary to ease the transition, that provide conversation from the strength wind speed and wind service speed of ASCE7-05 as shown below :

2-    Missing load combination :

The SBC mentioned the load combination in section 2.3.2. However, there is additional load for designing concrete and masonry mentioned in the exception clause No.2 which usually unobserved .

3-    Design coefficients and factors for basic seismic force-resisting system 

SBC code is using lower value of the response modification coefficient(R), seismic over-strength factor and defection amplification factor Cd than IBC code .

New version of SBC is under reviewing and expected to be released by end of this year .