Case studies to determine the man-hours required to construct the concrete frames of tall buildings in summer and winter

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Case studies to determine the man-hours required to construct the

concrete frames of tall buildings in summer and winter

CANADA Ser T H l B92 no. 70 c. 2

CASE

STUDIES

TO

DETERMINE THE MAN-HOURS

REQUIRED

TO CONSTRUCT

THE CONCRETE FRAMES

O F TALL BUILDINGS

IN

SUMMER

AND

WINTER

D I V I S I O N OF B'UILDING R E S E A R C H N A T I O N A L R E S E A R C H C O U N C r t

-

O T T A W A C A N A D A May 1970

CASE STUDIES T O DETERMINE THE MAN-HOURS REQUIRED

T O

GONSTRWCT T H E CONCRETE FRAMES

O F TALL BUILDINGS IN SUMMER AND WINTER

by

J. K.

Latta

h

the early 1960% s the Canadian Construction Association con- ducted

a

survey t o determine the o v e r - a U e x h a cost of constructing buildings in winter as opposed t o summer. The r e s u l t s c a a be s u m m -

arized as follows:

F o r all contracts 0.97 p e r cent

F o r contracts over $660,000. 0.79 per cent

For contracts between $ 2 0 , 000 and $660,000 1. 31 per cent From the ownerls point of view, therefore, an early r e t u r n on his in-

vestment would probably m o r e than offset the extra expenditur e involved. F o r the contractor, however, the picture is not as clear.

I£

h e

carries out

an

operation in w i n t e r that requires any special protection not needed in summer then, a11 other things being equal, it is going to

c o s t him extra money.

If,

under the terms of

his

c o n t r a c t , he can d e - lay doing that operation until summer it will be to his f i n a n c i a l advantage

t s d o s o . T h e p r o b l e m f a c i n g t h e c o n z t r a c t u r i s t o d e t e r m i r r e h o w m u c l a

more the winter operation w i l l c o s t than the summer operation. _{B y}

summer, however, many conditions w i l l have c h a n g e d and so "all other things" never will be equal f o r the two conditions. A review of s a m e of the changes which c a n b e expected is being p r e p a r e d for future publication.

One of the variants that must b e taken into account is t h e number of man-hours required t o c a r r y out the _{operation under both summer} and winter conditions, Unfortunately, it would appear that there a x e no reliable f i g u r e s available f o r factors such as this. Thus the present study was undertaken in an effort to obtain some indication of the r ekative productivity of winter and summer operations.

The first problem

in

setting up t h e study was to select some we41 defined operation that might b e expected t o b e adversely affected by winter conditions. It also had t o be one that could readily be identified in the

contractors' r e c o r d s since it w a s not possible t o c a r r y out detailed d a y by day obs exvations with the staff available. F k a l l y s it had to be an operation that was done on the s a m e site by t h e s a n e contractor and, as f a as possible, by the s a m e workmen in both seasons.

One activity that meets all these requirements is the construction of the reinforced concrete frame of a tall building. W i t h the exception of

t h e ground f l o o r , the roof, and t h e mechanical equipment f l o o r s each floor is, w i t h minor exceptions, identical with all the o t h e r s . It is possible, ther efoxe, to g e t a repetitive a c t i v i t y that can b e clearly identified.

T h e

contrac tor1 s concreting r e c o r d g i v e s the s t a r t and finish of each c y c l e in t h e repetition although this is not always as

clear-cut as one would wish when a f l o o r i s c a s t in two or m o r e separate operations.

The information was gathered b y visiting the contractorls of - f i c e and extracting, from his time sheets, the number of hours worked

each day b y each man.

As

t h e s e time sheets a r e the ones used to pay the workmen they presumably can be taken to be reasonably accurate,

but, a c c u r a t e o r not, they a r e the basis on which the contractor pays f o r the work that i s done. The individual times w e r e then totalled to g i v e the n u m b e r of man-hours each t r a d e worked on each day. The daysf activities were obtained f r am t h e general c o n t r a c t o r s f records,

thus the man-hours expended in f o r m i n g , r e i n f o r c i n g and casting each f l o o r or group of f l o o r s could be calculated. Because of different de- signs, site conditions, methods of operation and times of construction, no comparison can be made betweendifferent jobs. Comparison is limited to the s a m e group of workmen doing t h e same operation on the same building in summer and in w i n t e r .

Three different buildings w e r e studied, Two w e r e office build-

ings with a central elevator s e r v i c e core and flat slab f l o o r s , t h e other was an apartment building with an elevator bank at one side, a stair-

well near each end and flat slab floors. For convenience in identifying the buildings t h e y w i l l be designated O f f i c e Building A, Office Building B, and Apartment

C.

In

assessing t h e results i t is necessary to exercise some care and t o eliminate, as far as possible, various outside influences. The

fir st two or three floors of a tower cannot reasonably be taken as typical for site operations because the crews are making up the necessary sets

of f o r m w o r k and learning the routine. It is normal, on most sites for the floor slab to b e c a s t in two or mare sections. This c r e a t e s an o v e r - lap in some activities in t h a t t h e floor above will be being formed at one end of t h e building while t h e lower f l o o r is being c a s t . This makes it

difficult to allocate the man-hours worked at that time to the correct f l o o r .

W h e r e a f l o o r i s c a s t as one operation it gives a definite change point in the activities because no work can be s t a r t e d on the next floor until the f r e s h concrete has s e t .

E

such a clear-cut change point is not available the next b e s t is one where the floor was c a s t on two successive working days.

O F F I C E BUILDING A

Office Building A is a 20-storey building w i t h a tower r i s i n g above

a bas e podium of 2 s t o r i e s . Floors 3 t o 20 are typical f l o o r s w i t h a r o o f and penthouse above the 20th floor. The typical f l o o r s a r e 7 5 x 170 f t with

a

centre c o r e 2 6 x 66 f t 6 in.

Of t h e 18 f l o o r s of typical layout floors 3, 4 and 5 must be o m i t t e d

since the standard routine of construction is not established until f l o o r 6.

Of the r ernaining f l o o r s reasonably clear -cut s t e p s in the progress of the job a r e p r o v i d e d by f l o o r s 6 , 8 , 11, 14, 18 a n d 20. F l o o r 6 and 20 were each

c a s t in one day and s o the constructionof f l o o r s 7 to 2 0 inclusive w i l l be taken a s typical construction operatiolls. The t o t a l man-hours required to

p r o d u c e one f l o o r w a s then calculated for these steps and f r o m this f i g u r e the total man-hours required t o p r o d u c e one square foot of net f l o o r a r e a , to cast one cubic y a r d of c o n c r e t e , and to e r e c t one square foot of f o r m -

work, w e r e calculated. T h e s e r e s u l t s a r e summarized in Table

I.

TABLE

e

The surprising thing s h o w n by these f i g u r e s is that the greatest l a b o u r output w a s r e a l i z e d on f l o o r s 1 5 to 20 which w e r e constructed during a period when the weather w a s bad, t h e job w a s i n t e r r u p t e d by the C h r i s t m a s and New Year's holidays and w a s c l o s e d down f o r a week, in J a n u a r y , be -

cause of e x t r e m e l y severe weather.

O F F I C E BUILDING B

Office Building B is a d u a l tower structure w i t h both t o w e r s r i s i n g

f r o m a common plaza at ground l e v e l . O n e t o w e r ( T o w e r S) w a s built in s u m m e r and i s a 1 7 -storey structure w i t h a machine room a n d penthouse

above the 17th floor level. The column height from the 1 s t t o

2nd

floor

is g r e a t e r than f o r the other f l o o r s thus only floors 3 to

17

inclusive can be considered typical. The floors are 8 8 E t l i n x 1 9 3 f t 1 0 in.in avex-all dimensions w i t h a centre core 27 f t b in.x 61 f t 0 in.

The other tower ( T o w e r W) w a s built in w i n t e r and

is a

1 9 - s t o r e y

s t r u c t u r e with a machine room and penthouse above the 19th f l o o r . F l o o r s

3 to 19 inclusive can b e considered typical; they are 88 f t I in, x 153 5: l 0 in. in over-all dimensions and have a central c o r e 27 ft 6 in.x 61 f t 0 in. The reduction in the length af Tower W compared with Tower S is made by e l i m -

inating one bay on each s i d e of the centre core.

Suitable divisions to r e c o r d the progress of work on both t o w e r s

w e r e afforded by the completion of floors 5, 9 a n d 14, It is purely fortuitous

that the same floors a r e used on both towers. Above f l o o r 14 on T o w e r S there was some overlap with w o r k on the basement of Tower W which made

it impossible to allocate the man-hours t o one particular activity. It was noted from the concreting r e c o r d of Tower W that floor 1 5 w a s the first floor that d i d n o t require heat applied under it. This then c a n reasonably be taken as the conclusion of the winter construction season. The r esults f o r both t o w e r s a r e summarized in Table II.

TABLE 11 Tower S Tower W Date of Construction 7 J u n e - 8 A u g 7 June - 5 July 6 July - 8 A u g Floors man-hours per f l o o r 6 - 1 4 6 _{- 9} 1 0 - 1 4 201 2 1909 2094 sq ft of formwork 0 . 0 9 6 6 0 . 0 9 1 7 0. 1 0 0 6 s q f t o f floor 0.126 0.119 0. 1 3 1 c u y d o f concrete 3 . 494 3. 300 3 . 6 5 0

It is interesting to note that while the l o w e r four floors took m o r e man-hours ta build in winter than in s u m m e r on two of the t h r e e units of measure, the upper f i v e took fewer man-hours in winter on all three.

D u r i n g construction of the summer -built l o w e r floor s the mean daily tern - perature averaged about 60" F but when the upper f l o o r s w e r e being con-

s t r u c t e d t h e temperature w a s about 10°F higher and on some d a y s w a s

over 80" I?. Thus the lower floors could b e s a i d to have been built under

the m o r e favour able conditions. In the w i n t e r -built t o w e r the l o w e r

f l o o r s w e r e c o n s t r u c t e d under the m o r e adverse conditions with mean daily temperatures ranging from zero to above freezing w i t h an average of 3 9 " F. The upper floors w e r e built under s l i g h t l y milder conditions with temperatures ranging between 10°F and 40°F a n d averaging

26°F.

It

is a f u r t h e r point of interest that on Tower W f l o o r s 6 to 24,

which required winter protection, took an average of 161 5 man -hours

to construct, whereas f l o o r s 1 5 to I7 inclusive, which did not

-

require

winter protection, took an average s f 1610 man-hours, i. e . , an increase

of 3. 4 per cent.

APARTMENT

C

Apartment. C is a 14-storey s t r u c t u r e w i t h all floors above the

ground being typical. f l o o r s . It is 286 f t 4 in.long by 5 5 ft 0 in. w i d e with an elevator tower projecting out at the centTe of one long s i d e . Originally, it w a s the developer's intention t o construct a second i d e n - tical building the following s u m m e r thus giving an i d e a l w i n t e r s u m m e r

cornparis on. Unfortunately, this s e c o n d building was not constructed.

However, a r e c o r d w a s kept of t h e man-hours spent on winter protection,

heating, snow clearing e t c .

,

d u r i n g construction of a p a r t m e n t C. It

w a s found that 2247$ man-hours w e r e spent on winter w o r k s w h i l e build- ing the top seven f l o o r s but that, during

the

s a m e time, 107442 m a n - h o u r s w e r e spent on production activities (forming, reinforcing, con -

cr eteing, etc, ) The winter works activities represent, therefore, 2 1

p e r cent of the productive activity.

C ONG

LUS

IONS

What conclusions a r e to be d r a w n f r o m these r e s u l t s ? T h e first is that t h e r e is no clear evidence, in this study, of a reduction in the out- put of the workman in c o l d weather.

K

anything the indication i s that they w o r k f a s t e r . One reason for this could be that they have t o d o s o t o k e e p

w a r m but a second and p o s s i b l y m o r e important one, could b e that the s i t e management is more concerned about the p o s s i b l e effects of a con - struction delay in c o l d weather andsoexercises tighter control over the

operations. There is some evidence in the r e c o r d s f o r Office Building B

that construction is slowest

in

the hot summer weather.

A second conclusion might be that the r e c o r d s kept by these con-

f i r m conclusions to be drawn from them. The results f r o m Apartment Building

C

showed that winter works required 21 per cent of the man- hours of the productive activity. If this is in any way a typical f i g u r e then one would expect that Tower W of O f f i c e Building B would r e q u i r e the s a m e amount of extra hours but the figures show no increase over

the summer -built tower despite the fact that the man -hours spent on

winter pratectian are included.

It

should be pointed out that t h e protective

enclosures provided on the apartment block were of a v e r y high quality and they w e r e w e l l heated whereas those for Office Block

B

w e r e com- paratively poor and were poorly heated. Nevertheless, there w a s extra work to be done which should have been reflected in the m a n - h o u r s r e -

quired. T h e fact that it w a s not c a n b e taken to mean either that t h e

men worked m o r e efficiently to compensate for the extra work or that

the nature of the study failed to reveal it.

Further studies to identify and determine the influence of all the

factors affecting productivity are required before an accurate assessment of the relative outputs of the labour f o r c e in s u m m e r and w i n t e r can b e established. Certainly some of the traditionally -held b e l i e f s regarding w i n t e r construction have not been confirmed in this lirn i t e d investigation.