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BEAMS

DESIGN EXAMPLE

The following consideration has been made:

A single span roof structure.  Hollowcore slabs spanning 17m supported on prestressed beams spanning 10m.

Loads:

Chippings g = 1.00 kN/m2
Insulation & roofing g = 0.25 kN/m2
Screed  g = 1.50 kN/m2
HCS 400mm g = 4.95 kN/m2
Live Load g = 0.75 kN/m2

Total characteristic load Pk

Pk = ½ x 17 (1.00+0.25+1.50+4.95+0.75) = 71.83 kN/m

Total ultimate design load Pd

Pd = ½ x 17 (1.4(1.0+0.25+1.50+4.95)+1.6 x 0.75) = 101.83 kN/m

Required beam size

From the load and deformation tables the following beams have sufficient capacity to sustain the load.

RB 36/90:        quls = 150.7 kN/m > Pd          
                        quls = 81.7 kN/m > Pk

RB 36/100:        quls = 158.0 kN/m > Pd          
                        quls = 93.5 kN/m > Pk

Assuming the load on the beams has little or no eccentricity a IB 36/100-beam would be the most economical as the self weight is only 75% of a RB 36/90-beam.  In case of eccentric load resulting in significant torsion on the beam as RB 36/90 would be a better choice.

Shear reinforcement shall be designed for the specific loads.

Deformation

The following deformations can be found using a IB 36/100-beam:

- Short term chamber:
            Csh = 8.8 mm

- Short term deflection for live load, Qk:
            ash = ½ x 17 x 0.75 x 1.4/10 = 0.9mm

- Short term deflection for dead load:
            ash = ½ x 17 (1.00+0.25+1.50+4.95) x 1.4/10 = 9.2mm

- Resulting short term deflection for dead load:
            a1= -8.8+9.2 = 0.4mm

Conclusion:

Elastic settlement for full live load is 0.9mm < span/130.

Combined short term deflection for dead load and camber is: 0.4mm < span/250.

Concentrated loads:

In case the beams are supporting concentrated loads the overall moment and shear capacity shall be checked.

Service and ultimate moment capacity, Msls and Muls can be derived from the load tables whereas the shear capacity shall be calculated in each individual case.



 

 

Columns

Design specifications

Columns are designed according to BS 8110.  The capacity of columns depends on the cross section, reinforcement, material properties, column length and combination of applied axial force and moment.  Reference is made to BS 8110, part 3 where design charts for rectangular columns are shown.

As an alternative, rectangular columns can be prestressed.  Prestressing is advantageous for slender columns subjected to relative high moments compared to axial load.  We can advise on the use of prestressing.

Connections:

Connections between column and column or column and beam are usually a pinned dowel connection where only axial and horizontal forces can be transferred.  Transfer of moments requires special design considerations.  Connection between column and foundation can either be a pinned dowel connection or a fixed.  Forming a pocket in the foundation where the column is inserted and grouted easiest does fixation.  For moderate loads a Precast foundation is a preffered solution.

Cross Section

RC-columns have a rectangular cross section and are designated RC w/h, where W is the width and h the height of the column.  The columns are produced in standard cross sections as per table below.  CC-columns have a circular cross section and are designated CC d, where d is the diameter.  The columns are produced in standard cross section as per table below.

RC - Column

h/mm
w/mm

200

240

300

360

420

480

600

700

200

 

 

 

 

 

 

 

 

240

 

 

 

 

 

 

 

 

300

 

 

 

 

 

 

 

 

360

 

 

 

 

 

 

 

 

420

 

 

 

 

 

 

 

 

480

 

 

 

 

 

 

 

 

600

 

 

 

 

 

 

 

 

700

 

 

 

 

 

 

 

 

CC-Column

h/mm

200

240

300

360

420

480

600

700

SPECIFICATIONS AND TOLERANCES

GENERAL MIX SPECIFICATIONS

Strength:
Min. characteristic cube strength fcu = 40 N/mm2 for normal reinforced beams and columns.  For prestressed beams  fcu = 60N/mm2.

Cement:
OPC complying with requirements in BS 12 and ASTM C 150, type 1.

Microsilica:
Densified or undensified Microsilica grade 920 or 940.

Water:
Clean water with total dissolved solid contents not exceeding 700 p.p.m.

Admixture:
Water reducing admixtures complying with ASTM C 494.

Coarse Aggregates:
Crushed aggregates complying with BS 882.

Fine Aggregates:
All fine aggregates are sand complying with BS 882.

Reinforcement:
All reinforcement steel complying with BS 4449.

Prestressing Strands:
All prestressing strands complying with either BS 5896, ASTM A416 or EN 138/79.

BEAM DIMENSIONS AND TOLERANCE

Dimensions                             Tolerances

Length, L                                + 12mm or L/1000*
Height, h                                 + 8mm
Width, w                                 + 8mm
Side camber, a                         +8mm or L/500*
Camber, Csh                            +50%
Position of cut-outs                 +10mm
Position of inserts etc.             +10mm

*whichever is the larger.

COLUMN DIMENSIONS AND TOLERANCES

Dimensions                             Tolerances

Length, L                                + 12mm or L/1000*
Cross section, w,h,d                + 10mm
Curvature, a                            + 5mm or L/750
Corbel position. Lc                 + 10mm
Position of cut-outs                 +10mm
Position of inserts etc.             +10mm