NATIONAL BUREAU OF STANDARDS REPORT

Test of the Mobilization Structure

Under Sustained Uniformly Distributed Load

by

L. F. Skoda

Report to

Bureau of Yards and Docks

Department of the Navy

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

7387
THE NATIONAL BUREAU OF STANDARDS

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July 1957 to June 1960 (Includes Titles of Papers Published in Outside Journals 1950 to 1959) (S2. 2 5); avail- able from the Superintendent of Documents, Government Printing Office, Washington 25, D. C.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1001-12-10411 November 27, 1961 7387

Test of the Mobilization Structure

Under Sustained Uniformly Distributed Load

by

L. F. Skoda

To

( Bureau of Yards and Docks

Department of the Navy

NATIONAL BUREAU OF STAND

intended for use within the Gov

to additional evaluation and revie

listing of this Report, either in w

the Office of the Director, Nation however, by the Government age

to reproduce additional copies fo

IMPORTANT NOTICE

'ess accounting documents Approved for public release by the b published it is subjected

Director of the National Institute of

^tion, or open-literature c . , , , „ , „ is obtained in writing from Standards and Technology (NIST) h permission is not needed, on October 9, 2015. >ared If that agency wishes

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U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Test of the Mobilization Structure

Under Sustained Uniformly Distributed Load

L. F. Skoda

A 44- by 80-ft precast thin-shell panel frame structure was constructed for the purpose of evaluating erection techniques,

structural stability and joint sealing materials. The problems

encountered during erection can be mainly attributed to inadequate dimensional control of the precast units and handling

techniques used after casting and during erection of the precast

units; the difficulties during erection of the structure could

have been avoided by strict adherence to more rigid dimensional

control.

A uniformly distributed live load of 40 psf was applied to

a two bent test section of the structure. A maximum deflection

of 2 in. occurred at mid-span of the roof sections 5 months after

application of the test load, as equilibrium was approached. Side

wall movements were significantly large only at the west wall,

with a maximum deflection of 0.4 in. evident at equilibrium 3

months after initiation of the test. Daily deflection variations

were observed after equilibrium was reached which can be attributed to environmental changes.

The joint sealing materials used in the structure achieved

their intended purpose but other sources of water penetration

were apparent. The construction bolts that were exposed to the

elements, particularly those used at the wall-roof connections,

were a source of leakage. The more damaging source of leakage

was through the transverse cracks that appeared in the webs of

the roof panels. The exact clause of these cracks has not been

precisely determined so that a solution to this problem is not

apparent at this time.

1. INTRODUCTION

Among the problems confronting the engineer in the design of thinshell precast concrete structures are. the problems concerning casting and

erection techniques, dimensional tolerances, watertightness of joints, and

deflections under sustained loads. In order to develop the required information, a 44- by 80-ft experimental "Mobilization Structure" was erected on the NBS grounds at the request of the Bureau of Yards and Docks

and a two-bent section of it was subjected to a sustained load test for

a period of a year. This report summarizes the results of the observations

made on the structure during the past year.
*
2 .

2. PROCEDURE

2.1 Description of Structure.

The "Mobilization Structure" illustrated in figure 1 is a precast

thin-shell concrete building composed of 20 bents, each bent being 4-ft

wide and spanning 44 ft. The bents are made up of two side wall and two

roof panels that are essentially channel shaped in cross section. Fabrication of the building elements and erection of the structure were accom- plished on the basis of a competitive bid contract.

The panels were fastened to one another by a system of bolted and

welded connections. Inserts had been cast into the wall panels and footings that enabled a welded connection to be made between each wall panel

and the footing. The knee joint (wall-roof connection) was made with lap

welded splices of metal plates that were attached to the main reinforcing

of each wall and roof panel prior to casting. The roof panels comprising

each bent were joined together at the crown with two 3/4 in. D bolts that

passed through transverse ribs located at the crown ends of the roof

panels. Each bent was connected to the previous bent with a number of

3/4 in. D bolts. These bolts passed through pipe sleeves that were precast into the longitudinal legs of all panels. Closure of the structure

was accomplished with end wall panels that were bolted to the first and

last bents. Views of wall and roof panels, and joints between them are shown in figures 1 through 6.

2.2 Design of Test

The third and fourth bents from the south end wall were selected as the two bent test section. These particular bents were chosen because

erection of the structure began at the north end wall, thereby enabling

the contractor to develop an erection technique prior to erection of the

test bents. The test section was purposely separated from the rest of

the structure by a space of 1/2 in. The two test bents were bolted together but no connections were made to the adjoining bents.

The live load of 20 psf was used in the design of the structure. A

uniformly distributed live load equal to twice the design load was applied

to the test section. The test load consisted of pea gravel of known weight

carefully screeded to a predetermined depth. The gravel was maintained at

a uniform depth by a plywood framework that surrounded the test section.

The plywood framework was arranged so that the two bents comprising the

test section could be loaded separately.
;

.
3.

2.3

Instrumentation

The displacements of the various points of the test panels were measured with 0.001 in. dial gages attached to pipe stands supported by

the concrete floor in the structure. Deflection measurements were made

at the quarter span and mid-span stations of the test section. Side wall

movements were measured at the knee joints 8 ft from the floor and 2 in.

above the wall floor interface. A sufficient number of gages were placed

at each station so that any differential movement between the two adjoining test bents could be detected. Reference gages were also placed on the unloaded panels adjacent to the test section.

Elevation measurements were made of the floor slab under the test

section with a Dumpy level and a Philadelphia rod. Bench marks were es- tablished on the footings at each side of the north end wall. A series

of five stations were established at points as near as possible to the

dial gage stands. Elevation measurements were made during the test to

determine any movement of the floor slab so that the observed dial gage

readings could be corrected.

In addition to deflection and elevation measurements, temperature

measurements were made at various locations on the structure. Thermometers were attached to the dial gage stands, to the bottom of the longitudinal legs and to the under side of the webs of the roof panels. Temperatures of the exposed roof surfaces adjacent to the test section were

also measured.

2.4

Application of Load

The 40 psf test load was applied in increments of 10 psf. The two

test bents were alternately loaded with 10 psf increments and deflection

readings were made after each increment. The bents were loaded alternately in order to observe any differences in deflection due to the non- symmetrical application of loads. After the entire test load was applied,

the gravel was completely covered with a polyethelyne membrane in an effort to maintain the gravel at a constant weight during any future

periods of inclement weather.

2.5

Collection of Data

The deflection measuring system was installed two weeks before the

test load was scheduled to be applied. A number of preliminary readings

were taken in order to establish the time of day that the deflections

caused by environmental changes were at a maximum. The results of these

readings indicated that the maximum deflection occurred at 11:30 A.M.

(EST) and it was decided to take all subsequent readings at this time.

4

At 11:30 A.M. on the day that the test load was applied, a set of zero readings were taken on all dial gages and elevation stations. The test load was then applied, as previously described, in 10 psf increments to alternate

bents with a complete set of readings being made before and after each increment. The total load of 40 psf was applied by 2:30 P.M. on the same day at

which time a final set of readings was taken. Subsequent readings were made

on all dial gages according to the following schedule: first 30 days, daily;

next 60 days, bi-weekly; next 9 months, bi-monthly; after the first year,

monthly.

The reference gages measured movements of the unloaded panels adjacent

to the test section. As thefie adjacent panels were bolted to the structure

along one edge only, it was felt that deflection measurements of unloaded

panels that were bolted along both edges would be more significant. Consequently, one month after application of the test load, additional gages

were placed at the crown and knee joints of a bent located halfway between

the test section and the north end wall.

Five months after the test began the deflection measuring system was modified. Mechanical difficulties necessitated the elimination of the

gages measuring the quarter span deflection and the gages measuring wall

movements 2 in. above the floor. The gages at the knee joints and crown were rearranged to measure average movements at each station rather than

movements of individual panels. The reference gages at the adjacent panels

and at the middle of the structure were allowed to remain undisturbed.

3. RESULTS

3.1 Instantaneous Deflections During the Loading Period

The differential deflections anticipated as a result of alternate bent

loading did not materialize. Figure 7 indicates the deflection vs. applied

load at the crown and quarter points. The applied load notation refers to

the amount of load applied to each bent (i.e., 0-10 means 0 psf on the north

bent and 10 psf on the south bent). Although these curves represent the

average deflection across the profile of each station, the load-deflection

curves were essentially identical for individual bents.

Side wall movements did not become significant until the entire live

load was applied. Measurements made at the kneee joints (8 ft above the

floor level) indicated an average outward movement of .045 in. at maximum

load. Measurements made 2 in, above the floor indicated an average outward

movement of .002 in. at maximum load.

5.

3.2 Deflections After One Year

Figure 8 is a graphic presentation of the deflection at the crown and

quarter points plotted against the duration of sustained load. The deflection at the crown increased at a rapid rate for the first 20 days. The

rate of increase became essentially linear for the next 100 days. From the

120 day period until the structure was under test for a year the deflection

at the crown increased at a rate of approximately .01 in. per month, with

a total one year deflection of 2.07 in.

The quarter point deflection measured just after application of the

40 psf load was .23 in. This initial deflection doubled after one day and

increased to .92 in. after 148 days. As previously mentioned, mechanical

difficulties necessitated the elimination of these gages after 148 days.

Side wall movements at the knee joints and footings are shown in

figure 9. Very little lateral movement was observed at the knee joint in

the east side wall, the maximum being .08 in. 135 days after the beginning

of the test. The west side wall stabilized to an average outward movement

of .4 in. approximately 3 months after the beginning of the test. Side

wall movements measured at the footings followed the displacement pattern

observed at the knee joints.

A comparison of movements at the crown of panels adjacent to the test

bent and panels near the center of the structure is made in figure 10. The

gages near the center of the structure were placed in service approximately

a month after the test began so that the comparison of deflections could

only be made from that time on. Movement of the panels adjacent to the

test bents agreed very closely with those observed near the center of the

structure. This agreement indicates that the entire roof section acts as a unit when subjected to environmental changes.

3.3 Effects of Snow Loads and Temperatures

The deflections observed at times when the structure was subjected to

various snow loads are indicated on figures 8 through 10. The deflection

observed during the first snow load of 7 In. shows an expected increase in

deflection at the crown accompanied by lateral movement of the side walls.

The second snow load of 6.5 in. was preceded by heavy rainfall and a sudden

freeze. This unusual moisture condition in the panels and formation of

ice in the open joint between the test bents and the rest of the structure

resulted in erratic movements at all gage stations. The third snow load of

7.5 in. was preceded by dry weather and the observed deflections conformed

to those normally expected. The snow loadings did not appear to produce any

lasting effects as deflection recovery occurrred after each thaw.

Temperature measurements were taken at various positions on the structure at the same time as deflection measurements. The difference in temperature of the roof surfaces and the underside of the longitudinal legs of

the panels have been as high as 30° or more. Although an attempt was made

6 .

to correlate these temperatures and temperature differentials with observed deflections, no direct correlation was found to exist between temperature and deflections. It appears that other factors such as changing

moisture conditions of the concrete must be considered.

4.

SUMMARY

1. The instantaneous deflections at maximum load of the crown,

quarter points and knee joints were .380, .236, and .045 in. respectively

with no differential deflections observed due to alternate bent loading.

2. The equilibrium deflection at the crown was reached 5 months after

application of the 40 psf test load and was approximately 2 in.

3. Side wall movements were only significantly large at the west wall

which attained a maximum stable deflection of 0.4 in. 3 months after initiation of the test.

4. Deflection measurements taken of unloaded panels at various locations within the structure show that the entire roof system acts as a unit

when subjected to environmental changes.

5. The effects of snow loading are temporary and full recovery was observed after melting.

6. Daily variations in deflection of precast concrete panels cannot be

attributed to temperature alone but must include the moisture condition of

the concrete.

5.

GENERAL OBSERVATIONS

A number of observations were made during and after erection of the

structure that were not directly related to the load test but are of sufficient importance to warrant discussion.

5.1 Dimensional Tolerances

It is noted that the firm responsible for casting the panels used in

the Mobilization Structure is a reputable concern with a great deal of

experience in precasting techniques. An inspection of the forms used to

cast the panels revealed that the firm did a commendable job of form con- struction with the personnel involved displaying a great interest in these

particular panels as well as the future of this type 6f construction. The

forms were constructed of sheet steel and structural shapes that were welded
'
7

together and adequately braced to prevent displacements during casting.

With all of the precautions taken to insure accuracy in casting, the

length of the completed structure was 2 in. in excess of the specified

length. If the observed departure of 2 in. in 80 ft, or 1 part in 480

is considered excessive, it is clear that specifications must be prepared which will require a higher degree of dimensional control than

is now available in a commercial plant experienced in precasting operations.

5.2 Erection of Precast Members

Another important point to be considered in the construction of a building such as the Mobilization Structure is the method used to move

the panels after casting and during erection. The lifting rig devised

to position the roof panels in the structure consisted of a stiff back

that was bolted to the panels by means of four threaded inserts that

had been cast into each panel. The side walls were positioned in a similar fashion with a stiff back bolted to the top of each panel by means of two threaded inserts. Figures 2 and 3 show panels being

positioned with a crane.

This system of moving the panels proved successful in that no panels were lost through breakage or severe damage. However, two objectionable features resulted from the use of this system. Cracks developed

across the webs of each roof panel between opposite pairs of inserts.

These cracks indicate that a plane of weakness was created by the insert

cavities. After erection of the building was completed the insert cavities were filled with dry grout. These grout plugs appeared sound prior

to the winter season, but after a number of cycles of freezing and thawing the bond between the panels and the plugs was destroyed to such an extent that the plugs could be simply lifted out of the cavities. These

results indicate that the panels must be adequately reinforced in the

vicinity of the embedded inserts, and that greater care must be taken

in grouting the insert cavities. In the future, the insert cavities

will receive a coating of epoxy/Thiokol adhesive prior to grouting, a system which proved to be successful in grouting welded joints of precast

concrete panels - (See NBS Report 6682).

5.3 Bolted Connections

The third point to be considered in this discussion deals with the

system of bolts used between bents. The first indication of an existing

problem was realized on the morning of the second day of erection. The

first three bents had been erected on the first day with all bolts in

place and only hand tightened. The following morning an inspection of

the three bent section revealed visible cracks between the vertical ribs

and the webs of the side wall panels. Further investigation revealed
.
8

that the bolts were so tight that they could not be easily loosened with a wrench. The conclusion reached was that the tightening of the bolts re- sulted from the temperature drop that occurred between 4:00 P.M. of the

first day and 8:00 A.M. of the second day. With this experience in mind

the erectors installed all subsequent bolts in the wall panels with a very

minimum of tightening.

Another problem brought about by the use of the bolt system is that

each bolt exposed to weathering is a source of water leakage during rainy

weather. The bolts at the knee joints in particular exhibited a tendency

to permit the passage of water. The reason for this is that water running

off the roof follows the contour of the upper wall section and passes

through the bolt holes. The same problem is evident at the bolted connections between wall panels but with less severity than was observed at the

knee joints. The elimination of exposed bolts would, of course, be the

ideal solution to the problem and future design considerations should be

directed towards this end.

5.4 Cracks in Webs of Roof Panels

It was observed a number of months after erection of the structure

that the webs of all roof panels exhibited transverse cracks spaced approximately 2 ft apart along their lengths. These cracks were large enough to

produce visible damp spots on the under side of the panels during any rain

storm. When a continuous light rain or a sudden heavy rain storm occurred

sufficient water leaked through the cracks to completely wet the floor area.

The first explanation advanced for these cracks was that they occurred

as a result of drying shrinkage. An investigation was undertaken to produce

similar cracking in scaled down laboratory size specimens similar to the

roof panels. Although this investigation is not complete, indications are

that shrinkage alone does not account for the formation of transverse and

longitudinal cracks in the webs. The present hypothesis is that these

cracks were caused by a combination of drying shrinkage and thermal stresses

resulting from temperature differentials between the longitudinal legs and

webs of the panels.

USCOMM-NBS-DC

FIG.

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MOBILIZATION

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U. S. DEPARTMENT OE COMMERCE

Luther H. Hodges, beaetury

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work.

In general, each section carries out specialized research, development, and engineering in the field indicated by

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front cover. WASHINGTON, D.C.

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