JAIC , Volume 39, Number 1, Article 12 (pp. to )
JAIC online
Journal of the American Institute for Conservation
JAIC , Volume 39, Number 1, Article 12 (pp. to )

SEISMIC STABILIZATION OF HISTORIC ADOBE STRUCTURES

WILLIAM S. GINELL, & E. LEROY TOLLES



6 6. SHAKING TABLE TEST RESULTS ON 1:5 SCALE MODELS

The principal end results of the tests are summarized in table 3. Typically, cracks were initiated in the adobe walls during test levels III or IV at shaking table acceleration values of 0.23 g and 0.28 g, respectively. Cracks continued to develop during test levels V and VI, after which the principal cracked block sections had been defined and their location then determined the nature of seismic performance of the model during the succeeding tests (test levels VII and above).

Table 3. Retrofit Configurations Tested and Final State of Models
Model SLa Walls Retrofit Test Levelc Acceld Displacemente Final State of Model 
7.4 N, E Upper horizontal strap 0.58 38 Collapsed 
   S, W Upper and lower horizontal straps     
7.4 N, E Bond beam and center cores 0.58 38 Remained standing after 
   S, W Bond beam, horizontal, and vertical straps    3 repetitions of test level X 
7.4 N, E Bond beam and center cores 0.58 38 Partial collapse of east wall 
   S, W Bond beam, center cores, and straps     
5.0 N, E Upper horizontal straps 0.58 38 Remained standing 
   S, W Upper and lower horizontal straps     
11.0 N, E Control (no retrofits) 0.32 +13 Collapse of east, south, and west walls 
   S, W      
11.0 N, E Bond beam and vertical straps VIII 0.48 25 Collapse of east wall 
   S, W Bond beam and local ties     
7b 5.0 N, E Upper and lower horizontal, and vertical straps 0.58 38 Remained standing 
   S, W Upper and lower horizontal straps and single vertical strap     
8b 7.4 N, E Lower horizontal and vertical straps 0.58 38 Remained standing 
   S, W Center cores and lower horizontal straps     
9b 7.4 N, E Control (no retrofits) VI 0.40 +19 Both gable and walls collapsed 
  S, W      

a SL = Wall height-to-thickness ratio

bbb Tapanco-design model, with attick and roof

c Final test level

d Shaking table acceleration level, g

e Shaking table displacement, centimeters

Unretrofitted models collapsed during test levels V or VI (see fig. 3). These test levels had an acceleration value of 0.32 to 0.40 g, and, more important, the peak displacements were approximately 16.5 to 24.8 cm in the prototype domain. In the prototype domain, thin walls (SL=11) were approximately 36 cm thick, moderate walls (SL=7.5) were 53 cm thick, and thick walls (SL=5) were 79 cm thick. Very high accelerations will not destabilize adobe walls if the frequency is high and the displacements are small. If the displacements are small, the base input may have large accelerations, but without large displacements, it is nearly impossible to destabilize the thick walls that are typical of many historic adobe buildings.

Lightly retrofitted models were able to withstand somewhat higher test levels (fig. 6). Thin walls, with a bond beam and local ties, collapsed out-of-plane during test level VIII but the in-plane wall, with the same retrofit system, did not collapse until test level IX. The thick walls with horizontal straps remained standing through test level while the moderate walls, with the same retrofit system, collapsed during test level X. Other models, with retrofit systems more complete than the lightly retrofitted models, showed greater resilience to extended, strong table motions. The addition of either vertical straps or center core rods greatly improved overall structural performance at the higher test levels, but the walls with center core rods sustained less damage through the test sequence than the walls with vertical straps only. Walls with horizontal and vertical retrofit measures continued to behave well through repetitions of test level X, and there were no indications of imminent collapse.

Fig. 6. Retrofitted 1:5 scale model (a) before test and (b) after test

The results of the retrofitted tapanco model tests demonstrated the dramatic improvement in stability afforded by the wall retrofits and the continuity-assuring roof-to-wall attachments. Especially significant were the stability and damage control improvements that were provided by the thin center core rods (fig. 7).

Fig. 7. Tapanoco model 1:5 retrofitted (a) before test and (b) after test