Steel Beam Design

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25 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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Composite Construction

Dr M Gillie

Composite Floors


Connection between steel and concrete


Bending strength greatly enhanced (c50%)


Stiffness much great. Can govern design


Material working much closer to yield strength

Typical Floor System

Composite Systems


Highly optimised


Can be proprietary eg. Slimflor, SlimDeck


Consist of several elements


Profiled metal deck


Concrete with mesh/reinforcement


Steel beams


Columns


Two stages to design


Construction (non
-
composite)


Final (composite)

Typical Floor Arrangement

Profiled metal

deck

Steel I
-
section

Mesh

reinforcement

Concrete floor

Floor slab spans between

secondary beams

Composite s
econdary

beams span between primary

beams (or columns
)

Composite

primary
beams

span
between

columns

Typical Composite System

c3m

c6
-
10m (cellular beams longer)

Floor slab spans between

secondary beams

How the Floor Works

c3m

c6
-
10m (cellular beams longer)

How the Floor Works

c 3m

Profiled concrete slab

spans between beams

I
-
sections

Ribs run left to right in slab

Assume simply
-
supported over beams

Steel deck

acts as tension

reinforcement

Mesh mainly

to prevent

cracking

Concrete in

compression

Steel in

tension

F
s

F
c

x
1

M
u
~
F
c
(x
1
/2+x
2
)

x
2

Composite s
econdary

beams span between primary

beams (or columns
)

Typical Floor System

c3m

c6
-
10m (cellular beams longer)

How the Secondary Beams Work

Design as simply

Supported (conservative)

Profiled concrete slab

Steel I
-
section

“Shear studs”

Steel
-
decking

not acting

How the Secondary Beams Work

Tension in steel

Zero forces at ends

Assume UDL


Zero force at ends


Largest force at point of peak moment


Therefore shear studs must transfer forces


“Shear flow” governs how much force is transferred at each point

Compression


in concrete

Simple view of Composite Capacity

σ
c

σ
s

b

D

h

F
c

F
s

Section in ultimate

bending state

Forces

Taking moments about line of F
c
gives M
ult
~
F
s
(D+h)/2

This assumes


Steel capacity sufficient to resist concrete web force


Full shear connection


Ignores profiles in decking

Composite Capacity


Effective
Width

b

b

b

b
eff

b
eff

b
eff

Effective width depends on



Loading



Point within span



Support conditions


Empirical guidance in design codes

Obviously b
eff
<=b

Effect of Effective Width

σ
c

σ
s

b

F
c

F
s

σ
c

σ
s

b

F
c

F
s2

σ
c

σ
s

b

F
c

F
s


Large effective width


NA in concrete


Common in secondary
beams


Small effective width


NA in steel (web or
flange)


Common in primary
beams


Simplest case


NA at interface

F
s1

Shear Flow (simple elastic)

F
s

F
c

x

Consider “cut”

through a composite

beam at position x

Moments about 0


elastic)

if
Linear

flow.
shear

-

force

of

change

of

(rate

d)
transferre

force

(total

)
(
d
wx
d
wL
dx
dF
x
L
d
wx
F
d
F
wx
x
wl
c
c
c







2
2
2
0
2
2
2
d

0

wL/2

w

How the Secondary Beams Work
(heavily loaded)

Shear flow or force in studs

Elastic condition, low load

(varying load in studs)

Plastic condition, high load

(all studs take equal load

except middle one)

Studs deform

Composite Capacity


Partial Shear
Connection

F
s

F
c

x

d

0

wL/2

w

F
t1

F
t2

F
t3

If
F
t1
+ F
t2
+ F
t3
<F
c

then “partial shear” connection


This means total force in concrete can not be transferred by shear studs to steel

Quite common due to no room for enough studs

Not a problem in practice.