presentation to interior designers

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Basics of Structural Engineering

Presentation to Interior Architecture Students

By Mark Sheldon, Technical Director, Aurecon

Aurecon

Offices: 87

Countries: 28

Employees: 7,000+

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Who am I?

What we will cover

• Structural Systems

• Design Loads – what loads act on on a structure

• Design Actions – how does a structure react to these loads

• Impact of Interior Design on structure (and vice versa)• Setdowns in the floor

• Chases into the surface of the structure (eg to Boardroom tables)

• Operable walls

• Heavy point loads on floors or ceilings

• Lintels

• Transoms supporting wall lining

• Heavy duty shelves

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Structural Systems

• Lightweight steel roof

What it looks like on site What it looks like on a drawing

Structural Systems

• Lightweight steel roof

Design live load • Distributed Load (UDL) = 25kg/m2

• Point Load (P) = 100kg

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Structural Systems

• Flat slabWatch for thickenings around columns

• Design Live Load (office floor)• UDL = 300 – 500kg/m2

• P = 1000kg

What it looks like on a drawing

Structural Systems

• Band Beams

• Beams typically 2400mm wide and at around 8.1m centres

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Structural Systems

• Waffle slab

Structural Systems

• Bondek slab

• Steel Beams (secondary) around 2.8m centres

• Primary beams deeper!

• Slab typically around 120mm thick

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Structural Systems

• Timber (e.g. Mezzanine)

• Joists typically at 450mm centres

Design Actions

• Normal (actual) office loads approximately 100 – 300kg/m2

• (cf 300-500kg/m2 design load)

• Compactus units approximately 700kg/m2

• (cf 300-500kg/m2 design load)

• Ceilings approximately 15kg/m2

• Safes (smallish) approximately 1500kg

• Blockwork walls approximately 700kg/m length

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Design Actions

Consider the simple case of a single span with a Uniformly Distributed Load (UDL)

Examples will be a lintel over a new door opening, a beam spanning between columns, a stair stringer etc.

The structural engineer will draw it like this: -

He/she will then work out the stresses in the member to determine what size beam is required (or check whether the existing beam can take that load).

He/she will then determine how much the beam will deflect (sag) under that load to determine whether this can be accepted

Design Actions

Of course, the structure will rarely be this simple, so the structural engineer will typically use analysis tools such as: -

• Spacegass – for complex frames

• RAPT - for one-way reinforced concrete and post-tensioned concrete beams and slabs

• RAM Concept – for two-way concrete beams and slabs with complex geometry

• ETABS – for stability of tall buildings

• Strand7 – for detailed finite element analysis of portions of complex structures

• Grasshopper – interaction with Rhino and parametric modelling

• Numerous other programs

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Design Actions

0.125

Free Body Diagram

0.5

0.5

Consider the simple case of a single span with a Uniformly Distributed Load (UDL),

let’s look at some basic structural mechanics for this simple case: -

ReactionCoefficient

Stress coefficient

Don’t worry about the maths – just keep an eye on the color-coded boxes !

Red boxes refer to the stresses in the beam, and dictate the required size of the beam.

Blue boxes refer to the reactions, or the force that goes into the supporting columns or beams

Design Actions

• Single span (coefficients from previous page)

0.125

0.5

0.5

• Compare to coefficients for a double span

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Design Actions

• 3 span – note how coefficients change

Design Actions

• 3 span

• 4 span

Note how the coefficients change

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Design Actions

• Pattern loading

But …… loads aren’t always uniform

• Point loads (these coefficients are only if the loads are midspan!)

Design Actions

• Then there are cases such as cantilevers: -

• Beams and slabs often have both point loads and distributed loads

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Design Actions

…and then it starts to get complicated!!

Impact of Interior Design on structure

I want to cut this existing beam….

or

I want to cut a stair into this floor…..

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eg cutting a large hole in a slab• Existing BMD



• New BMD

0.1250.0700.070

0.0-0.125

1.25

0.37

5

0.37

5

0.5 0.5

0.0

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• New BMD

• Solution

0.1250.0700.070

0.0-0.125

1.25

0.37

5

0.37

5

0.5 0.5

Note: New beams may need services relocations!!

0.0

New Steel beam New Steel beam


“Can’t you make the beam shallower…. you can make it wider if you want”

The structural engineer needs to do a number of checks, but the most common two are for strength (bending stress) and deflection. Returning to our simple beam with a UDL: -

Bending stress (Beam depth)²

Deflection (Beam depth)3

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Structural Design

Beam 1

600mm deep x 300mm wide

Beam 2


Beam 1 is twice as deep, but has 4 times the strength in bending and 8 times the stiffness in bending (if beam 1 sags 10mm then beam 2 will sag by 80mm!)

Structural Design

Beam 1


Beam 3


Beam 3 has the same area, but Beam 1 still has 2 times the strength in bending and 4 times the stiffness in bending

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Small openings in slabs• Keep away from beams

• Can cut some reo bars but not Post-tensioning strands

• Can cut more reo bars if in non-critical zone for structure

• Same deal for

• Floor boxes

• Door closers


Setdowns in floors (eg new shower)• Reo only 20-25mm down and concrete cover required

for fire rating

• Can accept minor set downs above reo in tensile zones, not so easy in compressive zones (if you don’t know where these are – ask!)

• Many setdowns > 20mm deep are effectively an opening structurally

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Chases• Similar requirements for setdowns

• For ‘one way’ floor systems, may be able to do floor chases (rebates) parallel to the spanning direction but not possible perpendicular to span


Full Height Partitions / Posts etc

• Must allow deflection heads at top to allow differential movements

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Operable walls• Heavy weight (for the acoustic performance)

• Need steel frame above ceiling (services clashes)

• Needs stiff frame so wall doesn’t sag and jam


The existing floor won’t be flat• Codes allow around 6mm variation over 3mm on

finish of floor

• Codes allow around span/300 (i.e. 30mm for a 9m span) for deflection of floor

• Leveling screeds can be heavy and eat into load allowance

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The team

The effort by the team on an interiors project (eg a tenancy refurbishment) might be:-

• Interior Designer/Architect 50%

• MEP Engineer 25%

• Structural Engineer 5%

• Others (Bldg surveyor, 20%Fire Eng, ESD consultant)

But . . .

• Engage with the structural engineer early

• There are some things they will say can be done easily, whilst other apparently similar things can’t

• They may have only a small part, but if you don’t engage them up front they will seem like the worst person on your team!!

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